Talmud:
אָמַר רַב חִסְדָּא שׁוֹק בָּאִשָּׁה עֶרְוָה שֶׁנֶּאֱמַר גַּלִּי שׁוֹק עִבְרִי נְהָרוֹת וּכְתִיב תָּגֵל עֵרוּתְךָ וְגַם תִּרְאֶה חֶרְפָּתְךָ אָמַר שְׁמוּאֵל קוֹל בָּאִשָּׁה עֶרְוָה שֶׁנֶּאֱמַר כִּי קוּלָּךְ עֶרֶב וּמַרְאֵךְ נָאוָֹה אָמַר רַב שֵׁשֶׁת שֵׂעָר בָּאִשָּׁה עֶרְוָה שֶׁנֶּאֱמַר שַׂעֲרֵךְ כְּעֵדֶר הָעִזִּים:
Along these lines, Rav Ḥisda said: Even a woman’s exposed leg is considered nakedness, as it is stated: “Uncover the leg and pass through the rivers” (Isaiah 47:2), and it is written in the following verse: “Your nakedness shall be revealed and your shame shall be seen” (Isaiah 47:3). Shmuel stated: A woman’s voice is considered nakedness, which he derives from the praise accorded a woman’s voice, as it is stated: “Sweet is your voice and your countenance is alluring” (Song of Songs 2:14). Similarly, Rav Sheshet stated: Even a woman’s hair is considered nakedness, for it too is praised, as it is written: “Your hair is like a flock of goats, trailing down from Mount Gilead” (Song of Songs 4:1).
נְשַׁדֵּר לֵיהּ מַר שְׁלָמָא לִילְּתָא א"ל הָכִי אָמַר שְׁמוּאֵל קוֹל בָּאִשָּׁה עֶרְוָה אֶפְשַׁר ע"י שָׁלִיחַ א"ל הָכִי אָמַר שְׁמוּאֵל אֵין שׁוֹאֲלִין בִּשְׁלוֹם אִשָּׁה עַל יְדֵי בַּעֲלָהּ אָמַר לֵיהּ הָכִי אָמַר שְׁמוּאֵל אֵין שׁוֹאֲלִין בִּשְׁלוֹם אִשָּׁה כְּלָל שָׁלְחָה לֵיהּ דְּבִיתְהוּ שָׁרִי לֵיהּ תִּגְרֵיהּ דְּלָא נִישְׁוֵויךְ כִּשְׁאָר עַם הָאָרֶץ
Later on, Rav Naḥman suggested: Let the Master send greetings of peace to my wife Yalta. Rav Yehuda said to him: This is what Shmuel says: A woman’s voice is considered nakedness, and one may not speak with her. Rav Naḥman responded: It is possible to send your regards with a messenger. Rav Yehuda said to him: This is what Shmuel says: One may not send greetings to a woman even with a messenger. With her husband? Rav Yehuda said to him: This is what Shmuel says: One may not send greetings to a woman at all. Yalta, his wife, who overheard that Rav Yehuda was getting the better of the exchange, sent a message to him: Release him and conclude your business with him, so that he not equate you with another ignoramus.
אֵין שׁוֹאֲלִין בִּשְׁלוֹם אִשָּׁה - שֶׁמָּא מִתּוֹךְ שְׁאִילַת שָׁלוֹם יִהְיוּ רְגִילִים זֶה עִם זֶה ע"י שְׁלוּחָם וְיָבוֹאוּ לִידֵי חִיבָּה:
No extending greetings to a woman - lest through inquiry after her welfare they will become familiar with each other through their messenger, and come to love one another
רַבִּי יוֹסֵי בְּרַבִּי חֲנִינָא אָמַר כְּדֵי לְשַׁגֵּר לֵהּ כּוֹס שֶׁל בְּרָכָה תָּנֵי מִשּׁוּם רַבִּי יוֹסֵי לָמָּה נָקוּד עַל אַיּוֹ שֶׁבְּאֵלִיּוֹ לִימְּדָה תּוֹרָה דֶּרֶךְ אֶרֶץ שֶׁיִּשָּׁאֵל אָדָם בְּאַכְסַנְיָא שֶׁלּוֹ וְהָאָמַר שְׁמוּאֵל אֵין שׁוֹאֲלִין בִּשְׁלוֹם אִשָּׁה כְּלָל עַל יְדֵי בַּעֲלָהּ שֶׁאֲנִי
Rabbi Yosei, son of Rabbi Ḥanina, says: They inquired about her in order to send her the cup of blessing. It is taught in the name of Rabbi Yosei: Why are there dots in the Torah scroll upon the letters alef, yod, and vav in the word “to him [eilav]”? These letters spell ayo, which means: Where is he? The Torah is teaching the proper etiquette, which is that a person should inquire of his hostess about his host, just as he should inquire about the welfare of his hostess from the host. The Gemara asks: But doesn’t Shmuel say: One may not inquire about the welfare of a woman at all, as this is immodest? The Gemara answers: A greeting by means of her husband is different. Asking a husband about his wife is not considered immodest.
There is another talmudic source that is sometimes used as support for the limitations on woman's voice (Sotah 48a) in the context of a general ban on singing at feasts. Geonim use this ban to condemn having female entertainers at any gathering, regardless of whether they are singing or playing instruments. Nobody ever (until 1814) connects that ban to "kol isha" however, and it is relegated to a ban on music in general, and not women in particular, so I am ignoring it for the purposes of this chaburah.
Saul Berman:
...Most german scholars, foremost among them R. Eliezer b. Yoel Halevy, proceeded to indicate that given Samuel's law, one was prohibited from reciting the Shema while hearing the voice of a woman.
However, three divergent opinions were expressed by contemporaries and students of the Rabiah. R. Eliezer b. Samuel of Metz, author of the Sefer Yereim, extended the restriction to any Dabar shebekedusha; and R. Mordecai b. Hillel, citing R. Eliezer of Metz, implies its applicability to the study of Torah as well. R. Isaac b. Moses, in the Or Zarua, is the only authority who disagrees with the basic premise and denies the applicability of Samuel's law to the recitation of Shema. However, he fails to indicate what alternative applicability it might have.
Rabiah:
All of the things mentioned above as sexually stimulating (erwah) are only to be treated as such when they are not customarily exposed, but (for example, with regard to) an unmarried woman whose hair is customarily exposed, we need have no concern, for there is no arousal, as with regard to her voice, for one who is accustomed to hearing it."
אֲבָל פָּנֶיהָ יָדֶיהָ וְרַגְלֶיהָ וְקוֹל דִּבּוּרָהּ שֶׁאֵינוֹ זֶמֶר וְשַׂעֲרָה מִחוּץ לְצַמָּתָהּ שֶׁאֵינוֹ מִתְכַּסֶּה אֵין חוֹשְׁשִׁין לָהֶם מִפְּנֵי שֶׁהוּא רָגִיל בְּהֵן וְלָא טְרִיד. וּבְאִשָּׁה אַחֶרֶת אָסוּר לְהִסְתַּכֵּל בְּשׁוּם מָקוֹם וַאֲפִילּוּ בְּאֶצְבַּע קְטַנָּה וּבְשַׂעֲרָה, וְאָסוּר לִשְׁמוֹעַ אֲפִילּוּ קוֹל דִּבּוּרָהּ כִּדְאָמְרִינַן בְּקִדּוּשִׁין (ע, א) לִישְׁדֹּר מַר שְׁלָמָא לְיַלְתָּא אָמַר לֵיהּ הָכִי אָמַר שְׁמוּאֵל קוֹל בָּאִשָּׁה עֶרְוָה. וְאֶלָּא מִיהוּ נִרְאֶה דְּדַוְקָא קוֹל שֶׁל שְׁאֵלַת שָׁלוֹם אוֹ בַּהֲשָׁבַת שָׁלוֹם כִּי הָתָם דְּאִיכָּא קָרוֹב הַדַּעַת. וְהָרַב אַלְפָסִי זַ"ל שֶׁלֹּא הִזְכִּיר מִכֹּל זֶה כְּלוּם, כָּתַב הָרָאָבָ"ד זַ"ל דְּאֶפְשָׁר דְּמִשּׁוּם דְּאָמְרִינַן לְעֵיל עֲגָבוֹת אֵין בָּהֶן מִשּׁוּם עֶרְוָה, סָבַר הָרַב זַ"ל דְּכָל שֶׁכֵּן טֶפַח וָשׁוֹק וְשַׂעֲרָה וְקוֹל. וְכָתַב הוּא זַ"ל דְּלֹא מִן הַשֵּׁם הוּא זֶה אֶלָּא הָכָא מִשּׁוּם דְּמַטְרִיד וּבְרוֹאֶה, וַעֲגָבוֹת הָא פָּרֵישְׁנָא דַּוְקָא דְּנַפְשֵׁיהּ, וּבְאִשְׁתּוֹ בְּשֶׁאֵינוֹ רוֹאֶה וְאַף עַל פִּי שֶׁנּוֹגֵעַ, דְּכֹל שֶׁאֵינוֹ רוֹאֶה מִשּׁוּם נְגִיעָה לְבַד לָא מַטְרִיד הוֹאִיל וְגַס בָּהּ.
...But her face and her feet and her voice when it speaks and does not sing, and her hair that comes out of her braid and isn't covered, we aren't concerned about them because he is accustomed to seeing them and won't be distracted. And with another woman, it is forbidden to stare at her because of the place, and even her little finger or her hair, and forbidden to hear her even when she is speaking, as we said in Kiddushin 70a; "'Let the Master send greetings to Yalta,' and he said to him, 'This is what Shmuel says: the voice of a woman is nakedness'". And rather it appears that this is specifically the voice of sending or returning greetings, since the reason [of the prohibition?] is closeness of mind. And the Rav Alfasi of blessed memory, that he should not be recalled for this at all, wrote the Raavad of blessed memory that it is possible that this is because we said, see the letters, that this is not because of nakedness. The Rav of Blessed Memory thought this is all because a handbreadth of her thigh and hair and voice is nakedness. And he of blessed memory wrote that this is not a good reason, rather in this case it is because of distraction and because there is seeing, ... and even if he is touching [them], and everything he does not see but there is only touching, he is not distracted he is "arrogant" [familiar] with her.
אֵשֶׁת אִישׁ אֲפִילּוּ שְׁאִילַת שָׁלוֹם וַאֲפִילּוּ עַל יָדִי שָׁלִיחַ וַאֲפִילּוּ עַל יָדִי בַּעֲלָהּ אָסוּר, אֶלָּא לְמִי שֶׁיּוֹדֵעַ בְּעַצְמוֹ שֶׁאֵין תַּרְבּוּת יִצְרוֹ חָשׁוּד בְּסֶרֶךְ הִרְהוּר עַל דְּבָרִים אֵלּוּ כְּלָל. עַל זֶה וְעַל כַּיּוֹצֵא בָּזֶה נֶאֱמַר וְיִרְאַת מֵאֱלוֹקֶיךָ אֲנִי ה'
Meiri
A married woman - inquiring about her well being, even through a messenger, and even through her husband is forbidden but to a person who knows of himself that the character of his inclination is not such as to lead him to become habituated to sexual arousal through such matters. But concerning this and other such matters the Torah says 'you shall fear your Lord, I am God.
Saul Berman:
The result of all this is that R. Joseph Karo records in his Shulhan 'Aruk three separate laws deriving from the initial statement of Samuel, each of which is the exclusive opinion of one or more separate Rishonim, but to all of which no prior Rishon had consented.
1. He follows the Tosafists, Rabbenu Asher, and the Tur in prohibiting the sending of warm regards to a woman, even via her husband."
2. He follows the Rambam and the Tur in treating a woman's speaking voice as a possible "approach" to intercourse, therefore banning conversation with a woman who is an erwah to the particu lar man.".
3. He tends toward the position of the Rabiah and other Ger man Rishonim in preferring the avoidance of hearing a woman's singing voice while reciting the Shema's
The validity of each of these three laws depends upon totally different interpretations of the two basic sugyot in Kiddushin and Berakot. As well, they are reflections of interpretations of Samuel's law which are at total variance from each other both as to the context of its applicability and as to the form of the voice
יֵשׁ לִיזָּהֵר מִשְּׁמִיעַת קוֹל זֶמֶר אִשָּׁה בִּשְׁעַת ק"ש הָגָה וַאֲפִי' בְּאִשְׁתּוֹ אֲבָל קוֹל הָרָגִיל בּוֹ אֵינוֹ עֶרְוָה [ב"י בְּשֵׁם אוֹהֶל מוֹעֵד והג"מ]:
One should refrain from hearing a woman's singing voice during the reading of the Shema. and even his wife. But the voice that is regular to him is not Erva (lit. nakedness).
(יז) זֶמֶר אִשָּׁה - אֲפִילּוּ פְּנוּיָה אֲבָל שֶׁלֹּא בִּשְׁעַת ק"ש שָׁרִי אַךְ שֶׁלֹּא יְכַוֵּין לֵהָנוֹת מִזֶּה כְּדֵי שֶׁלֹּא יָבוֹא לִידֵי הִרְהוּר וְזֶמֶר אֵשֶׁת אִישׁ וְכֵן כָּל הָעֲרָיוֹת לְעוֹלָם אָסוּר לִשְׁמוֹעַ וְכֵן פְּנוּיָה שֶׁהִיא נִדָּה מִכְּלָל עֲרָיוֹת הִיא וּבְתוּלוֹת דִּידַן כּוּלָּם בְּחֶזְקַת נִדּוּת הֵן מִשֶּׁיַּגִּיעַ לָהֶן זְמַן וֶסֶת. וְקוֹל זֶמֶר פְּנוּיָה נׇכְרִית הִיא ג"כ בִּכְלַל עֶרְוָה וְאָסוּר לִשְׁמוֹעַ בֵּין כֹּהֵן וּבֵין יִשְׂרָאֵל. ומ"מ אִם הוּא בַּדֶּרֶךְ בֵּין הָעַכּוּ"ם אוֹ בְּעִיר וְהוּא אָנוּס שא"א לוֹ לִמְחוֹת כֵּיוָֹן דְּלָא מָצִינוּ דְּמִקְּרֵי עֶרְוָה מִדְּאוֹרָיָית' מוּתָּר לִקְרוֹת וּלְבָרֵךְ דאל"כ כֵּיוָֹן שֶׁאָנוּ שֶׁרַוְיִין בֵּין הָעַכּוּ"ם נִתְבַּטֵּל מִתּוֹרָה וּתְפִלָּה וע"ז נֶאֱמָר עֵת לַעֲשׂוֹת לַד' הֵפֵרוּ תּוֹרָתֶךָ אַךְ יִתְאַמֵּץ לִבּוֹ לְכַוֵּין לְהַקְּדוּשָּׁה שֶׁהוּא עוֹסֵק וְלֹא יִתֵּן לִבּוֹ לְקוֹל הַזֶּמֶר:
The singing voice of a woman, even an unmarried one, is permitted not during kriat shma as long as one isn't intending to enjoy it so that he won't be lead to improper thoughts. But a married woman's singing voice and any other woman he's forbidden from having sexual relations with - are forever forbidden. The same applies to an unmarried woman in nida and virgins (who are assumed to be nida). And an unmarried gentile woman's voice is also considered "erva" and is forbidden to listen to whether one is a Priest or a layman.However, if one is traveling with non Jews, or is in the city but is in the halachic category of an אנוס, he is allowed to read shma and make brachot, because [if we didn't paskin this way] Torah will become non-existent as we are living amongst the other nations.It has been said on this "time to do for God, by nullifying the Torah" However, one should put in an effort to focus on kedusha, and not pay attention to the singing
כְּשֵׁם שֶׁאָסוּר לִקְרוֹת כְּנֶגֶד צוֹאָה וּמֵי רַגְלַיִם עַד שֶׁיַּרְחִיק כָּךְ אָסוּר לִקְרוֹת כְּנֶגֶד הָעֶרְוָה עַד שֶׁיַּחֲזִיר פָּנָיו. אֲפִלּוּ כּוּתִי אוֹ קָטָן לֹא יִקְרָא כְּנֶגֶד עֶרְוָתָן אֲפִלּוּ מְחִצָּה שֶׁל זְכוּכִית מַפְסֶקֶת הוֹאִיל וְהוּא רוֹאֶה אוֹתָהּ אָסוּר לִקְרוֹת עַד שֶׁיַּחֲזִיר פָּנָיו. וְכָל גּוּף הָאִשָּׁה עֶרְוָה לְפִיכָךְ לֹא יִסְתַּכֵּל בְּגוּף הָאִשָּׁה כְּשֶׁהוּא קוֹרֵא וַאֲפִלּוּ אִשְׁתּוֹ. וְאִם הָיָה מְגֻלֶּה טֶפַח מִגּוּפָהּ לֹא יִקְרָא כְּנֶגְדָּהּ:
Just as the Shema may not be read in a place where there is ordure or urine till he has moved away [to a distance of at least four cubits], so it is forbidden to read the Shema in the presence of any person, even a gentile or child, whose privy parts are exposed, even though a glass partition separates him from them, unless he turns away his face. Since he is able to see, he may not read the Shema, unless he turns away his face. Any part of a woman's body falls under the same rule. Hence, while reading the Shema, one must not gaze at a woman's body, even if she is his wife. And if a hand-breadth of a part of her body [which is usually covered] is exposed, he must not read the Shema while facing it.
הָעוֹשֶׂה דָּבָר מֵחֻקּוֹת אֵלּוּ הֲרֵי הוּא חָשׁוּד עַל הָעֲרָיוֹת. וְאָסוּר לָאָדָם לִקְרֹץ בְּיָדָיו וּבְרַגְלָיו אוֹ לִרְמֹז בְּעֵינָיו לְאַחַת מִן הָעֲרָיוֹת אוֹ לִשְׂחֹק עִמָּהּ אוֹ לְהָקֵל רֹאשׁ. וַאֲפִלּוּ לְהָרִיחַ בְּשָׂמִים שֶׁעָלֶיהָ אוֹ לְהַבִּיט בְּיָפְיָהּ אָסוּר. וּמַכִּין לַמִּתְכַּוֵּן לְדָבָר זֶה מַכַּת מַרְדּוּת. וְהַמִּסְתַּכֵּל אֲפִלּוּ בְּאֶצְבַּע קְטַנָּה שֶׁל אִשָּׁה וְנִתְכַּוֵּן לֵהָנוֹת כְּמִי שֶׁנִּסְתַּכֵּל בִּמְקוֹם הַתֹּרֶף. וַאֲפִלּוּ לִשְׁמֹעַ קוֹל הָעֶרְוָה אוֹ לִרְאוֹת שְׂעָרָהּ אָסוּר:
One who engages in these behaviours is suspected of committing Arayos. And it's forbidden for a person to intimate with his hands or feet or to hint with his eyes to any of the Arayos or to laugh with her or to engage in light-headedness. And even to smell her perfume or to gaze at her beauty is forbidden. And one who engages in this deliberately receives lashes of rebelliousness. And one who gazes even at the little finger of a woman intending to derive sexual pleasure is comparable to one who looks at her genitalia. And even to hear the voice of an Ervah or to look at her hair is forbidden.
Rambam here is essentially not agreeing with Samuel's law. He is not saying a woman's voice is ervah and so is prohibited. He is only saying that it is forbidden to listen to a woman you are specifically forbidden to have relations with (not including wife or virgin). The ban is only in the context of the relationship one can have with an ervah, not that the voice itself constitutes ervah
Saul Berman:
For the Aḥaronim, however, neither distraction from due attentive ness nor the likelihood of illicit sexual intercourse resulting is the basic thrust of Samuel's law. For them, Kol b'isha erwah is a declaration that a woman's singing voice, under all circumstances, is to be considered a form of nudity, to be exposed exclusively to one's husband. In light of this proposition, it is understandable that the Aharonim virtually totally discard the limiting principle of accustomedness which the Rishonim used so extensively." One might suggest that being accustomed to a woman's voice limits its distracting quality, or limits the likelihood of its arousing someone to perform an illicit act of intercourse. But the Aharonim, in effect, suggest that being accustomed to seeing a woman's nudity in no way makes the act itself permissible.
Divrei Hefets Rav Aharon Toledo Salonika 1798 (quoted by Sde Chemed)
Sedei Hemed states that while most posekim forbid it, the author of Divrei Hefets permits it “so long as it is not a voice of lust-provoking songs and the listener does not intend to derive pleasure from her voice” and “there is no issue so long as he does not intend to derive pleasure from her voice.”
Conclusion of Sde Chemed: “One who sees [Divrei Hefets’] words… will rightfully deem them cogent. And even though it is surely correct to act stringently not in accordance with the aforementioned words of Divrei Hefets, in any case [they] are not, Heaven forbid, classified as inscrutable words.”
5b. Summary of the position of Rav Yechiel Yakov Weinberg (Seridei Esh 2:8) by Rav Chaim Jachter
Rav Yechiel Yaakov Weinberg (Teshuvot Seridei Eish 2:8) notes that traditionally women refrained from singing Zemirot when there were males who were not family members sitting at the Shabbat table. However, he records that the practice in Germany was for woman to sing Zemirot in the company of unrelated men. Rav Weinberg records that Rav Azriel Hildesheimer and Rav Samson Raphael Hirsch (two great German Rabbis of the nineteenth century) sanctioned this practice. Rav Weinberg reports that they based their ruling on the Talmudic rule (Megila 21b) that “Trei Kali Lo Mishtamai,” two voices cannot be heard simultaneously.
Rav Weinberg writes that he does not find this explanation satisfying (perhaps because the Gemara (Sotah 48a) writes that men and women singing together is a major impropriety). Rav Weinberg instead defends the German Jewish practice by citing the Sdei Chemed (Klalim, Maarechet Hakuf, 42) who quotes the Divrei Cheifetz who asserts that the Kol Isha prohibition does not apply to women singing Zemirot, singing songs to children, and lamentations for the dead. This authority explains that in these contexts men do not derive pleasure from the woman’s voice. In fact, the Pasuk (Shoftim 5:1) records that Devora the prophetess sang a song of praise to Hashem together with Barak the son of Avinoam. According to the simple reading of the text, Devora was married to Lapidot and not Barak. The Sdei Chemed writes that he believes that it is proper to be strict and not follow the approach of the Divrei Cheifetz, but he regards the lenient opinion as a viable approach.
Rav Weinberg writes that we should not pressure women who wish to follow the traditional practice to join Zemirot in a mixed group. Indeed, many Poskim oppose this practice of German Jewry (see Otzar Haposkim E.H. 21:1:20:3). However, some cite the Gemara (Megila 23a) that states that women are forbidden to receive an Aliyah to the Torah because of Kavod Hatzibbur as proof to the German practice. They argue that the fact that the Gemara does not mention Kol Isha as the reason to forbid women’s Aliyot proves that the Kol Isha restriction does not apply when a woman sings sacred texts. Others reply that the Gemara might be speaking of a woman reading the Torah to her immediate family members or may be speaking of a female child reading the Torah (see comments of Rav Shlomo Zalman Auerbach, Rav Eliezer Waldenberg, and Rav Yosef Shalom Eliashiv cited in Nishmat Avraham 5:76-77). These suggestions might also explain the Gemara (Berachot 57b and Rashi s.v. Kol) that states that hearing a woman’s voice is a soothing experience
Conclusion of Rav Moshe Lichtenstien
Under circumstances in which the song does not arouse sexual desire, does not emphasize femininity in a sensual manner, and the listener estimates that he will not come to have sexual thoughts – we should not forbid listening to a woman’s voice, whether in speech or in song. This conclusion not only relies upon the explicit stance of the greatest of the Rishonim – Rambam, Rashba and Ra’aviah; it appears in the literature of the Aharonim as a recognized opinion, and it has been applied in our generation by an eminent posek. This opinion takes into account the present societal reality together with its needs and constraints, while at the same time rules stringently regarding the obligation to preserve man’s dignity and embolden his image as a spiritual creature who is not controlled by biological drives alone.