אמר שמואל קול באשה ערוה שנא' (שיר השירים ב, יד) כי קולך ערב ומראך נאוה
Samuel said : A woman's voice is to be regarded as nakedness ; as it is said, "For sweet is thy voice, and thy countenance is comely" (Cant. ii. 14)
The context of this quote is in relationship to saying the Shm'a.
קולך ערב - מדמשבח לה קרא בגוה שמע מינה תאוה היא:
The very fact that her voice is praised here suggests that it is desirous.
Rav Hai Goan
This prohibition ONLY applies when a man is saying Kriyat Sh'ma, because a woman singing will distract him.
רא"ש על ברכות פרק שלישי - סימן לז
אמר שמואל (קיז) קול באשה ערוה שנאמר כי קולך ערב פירוש לשמוע ולא לענין ק"ש
Rosh On Berachot Chapter 3 Siman 37
The explanation of this text is to listen to a woman's voice, not for the matter of reciting the Shm'a.
חדושי הרשב"א על מסכת ברכות דף כד/א
ואסור לשמוע אפילו קול דבורה כדאמרינן בקדושין (ע, א) לישדר מר שלמא לילתא, אמר ליה הכי אמר שמואל קול באשה ערוה. ואלא מיהו נראה דדוקא קול של שאלת שלום או בהשבת שלום כי התם, דאיכא קרוב הדעת. והרב אלפסי ז"ל שלא הזכיר מכל זה כלום
It is even forbidden to hear a woman speak, as we say in Kiddushin....However it is specifically talking in a situation where one is creating familiarity. However, the Rif has NOT mentioned this law at all.
רמב"ם יד החזקה הלכות איסורי ביאה פרק כא
(ב) ... ואסור לאדם לקרוץ בידיו וברגליו או לרמוז בעיניו לאחת מן העריות או לשחוק עמה או להקל ראש ואפילו להריח בשמים שעליה או להביט ביפיה אסור ומכין למתכוין לדבר זה מכת מרדות והמסתכל אפילו באצבע קטנה של אשה ונתכוון להנות כמי שנסתכל במקום התורף ואפילו לשמוע קול הערוה או לראות שערה אסור:
Rambam on Forbidden Relationships
It is forbidden to gesticulate with his hands or his legs, or to hint with his eyes to a woman who is forbidden to him, or to laugh with her, or to act in a flirting manner, even to smell her perfume, or to look at her beauty. This is a rabbinic prohibition, and would incur Rabbinic lashes for someone who intends to do this....... and even to hear the voice of a forbidden women or to see her hair is forbidden.
(א) צָרִיךְ אָדָם לְהִתְרַחֵק מֵהַנָּשִׁים מְאֹד מְאֹד. וְאָסוּר לִקְרֹץ בְּיָדָיו אוֹ בְּרַגְלָיו וְלִרְמֹז בְּעֵינָיו לְאֶחָד מֵהָעֲרָיוֹת. וְאָסוּר לִשְׂחֹק עִמָּהּ, לְהָקֵל רֹאשׁוֹ כְּנֶגְדָּהּ אוֹ לְהַבִּיט בְּיָפְיָהּ. וַאֲפִלּוּ לְהָרִיחַ בִּבְשָׂמִים שֶׁעָלֶיהָ אָסוּר. וְאָסוּר לְהִסְתַּכֵּל בְּנָשִׁים שֶׁעוֹמְדוֹת עַל הַכְּבִיסָה. וְאָסוּר לְהִסְתַּכֵּל בְּבִגְדֵי צִבְעוֹנִים שֶׁל אִשָּׁה שֶׁהוּא מַכִּירָהּ, אֲפִלּוּ אֵינָם עָלֶיהָ, שֶׁמָּא יָבֹא לְהַרְהֵר בָּהּ. פָּגַע אִשָּׁה בַּשּׁוּק, אָסוּר לְהַלֵּךְ אַחֲרֶיהָ, אֶלָּא רָץ וּמְסַלְּקָהּ לַצְּדָדִין אוֹ לְאַחֲרָיו. וְלֹא יַעֲבֹר בְּפֶתַח אִשָּׁה זוֹנָה, אֲפִלּוּ בְּרִחוּק אַרְבַּע אַמּוֹת. וְהַמִּסְתַּכֵּל אֲפִלּוּ בְּאֶצְבַּע קְטַנָּה שֶׁל אִשָּׁה וְנִתְכַּוֵּן לֵהָנוֹת מִמֶּנָּה, כְּאִלּוּ נִסְתַּכֵּל בְּבֵית הַתֹּרֶף (פי', עֶרְוָה) שֶׁלָּהּ. וְאָסוּר לִשְׁמֹעַ קוֹל עֶרְוָה אוֹ לִרְאוֹת שְׂעָרָהּ. וְהַמִּתְכַּוֵּן לְאֶחָד מֵאֵלּוּ הַדְּבָרִים, מַכִּין אוֹתוֹ מַכַּת מַרְדּוּת. וְאֵלּוּ הַדְּבָרִים אֲסוּרִים גַּם בְּחַיָּבֵי לָאוִין:
(1) A person must stay very far from women. He is forbidden to signal with his hands or his feet, or to hint with his eyes, to one of the arayos. He is forbidden to be playful with her, to be frivolous in front of her, or to look upon her beauty. Even to smell the perfume upon her is forbidden. He is forbidden to gaze at women doing laundry. He is forbidden to gaze at the colorful garments of a woman whom he recognizes, even if she is not wearing them, lest he come to have [forbidden] thoughts about her. If one encounters a woman in the marketplace, he is forbidden to walk behind her, but rather [must] run so that she is beside or behind him. One may not pass by the door of a promiscuous woman [or: a prostitute], even four cubits [around 6–8 ft or 2–2.5 m] distant. If one gazes even at the little finger of a woman with the intent to have pleasure from it, it is as though he gazed at her shameful place. It is forbidden to listen to the voice of an erva or to look at her hair. If one intentionally does one of these things, we give him lashes of rebellion. These things are also forbidden in the case of ordinary Biblical prohibitions.
Rabbi Shimshon Refael Hirsch allowed men and women to sing together Shabbat songs. The reason that they gave is because two voices aren't able to be distinguished from one another, and since everyone is singing together, there is no problem here at all.
We see clearly that Devorah the judge sang together.with Barak. However, it could be argued that if this is based on divine decree then the laws of women singing where never said. TheCHIDAHof blessed memory states that when the heavenly presence resides on a woman,her singing is not forbidden.
It could also be argued that the whole purpose of the Rambam (source 6) is to emphasise that if one hasintention to enjoy or be aroused or get benefit from her voice. This idea is supported by the Meiri and by the ESHKOL. Those who are stringent would not allow a woman’s voice when it comes to Shm’a but they would certainly allow it if it was in a context where there is no intent to enjoy a woman’s voice...
The Beit Yosef himself states that even if a woman is singing, if a man is able to his Shm’a to the extent that he does not hear her, that he does not have to stop his reading of the Shm’a.