(1) Then Moses and the Israelites sang this song to the LORD. They said: I will sing to the LORD, for He has triumphed gloriously; Horse and driver He has hurled into the sea.
(א) אז ישיר משה. אָז כְּשֶׁרָאָה הַנֵּס עָלָה בְלִבּוֹ שֶׁיָּשִׁיר שִׁירָה....אַף כָּאן יָשִׁיר אָמַר לוֹ לִבּוֹ שֶׁיָּשִׁיר וְכֵן עָשָׂה ...לִמְּדָנוּ שֶׁהַיּוֹ"ד עַל שֵׁם הַמַּחֲשָׁבָה נֶאֶמְרָה,
(1) אז ישיר משה THEN SANG MOSES — with regard to the usage of the future ישיר, the meaning is: THEN — i. e. when he saw the miracle it entered his mind that HE WOULD SING a song...So, also, ישיר here signifies: his heart told him that he should sing, and thus did he actually do, as it states..This, too, teaches us that the י as a prefix of the imperfect is used in reference to intention to do a thing.
מיום שברא הקדוש ברוך הוא את העולם ועד שעמדו ישראל על הים, לא מצינו אדם שאמר שירה לקדוש ברוך הוא, אלא ישראל. ברא אדם הראשון ולא אמר שירה. הציל אברהם מכבשן האש, ומן המלכים ולא אמר שירה. וכן יצחק מן המאכלת, ולא אמר שירה. וכן יעקב מן המלאך, ומן עשו, ומן אנשי שכם, ולא אמר שירה. כיון שבאו ישראל לים, ונקרע להם, מיד אמרו שירה לפני הקדוש ברוך הוא, שנאמר: אז ישיר משה ובני ישראל.
From the day that God created the world until Israel stood by the sea, there was not a single person who said a song to God except for Israel. God created the first human and he didn't say a song. God saved Abraham from the firey furnace and from the kings and he didn't say a song. And also Isaac [was saved] from the knife and he didn't say a song and Jacob [was saved] from the angel and from Esau and from the men of Shechem and he didn't say a song. When Israel came to the sea and it parted for them, they immediately said a song before God, as its says Then Moses sang and the people of Israel.
Rebbe Nachman:
A holy melody has the power to bring one to the level of prophecy. Music is the foundation of true attachment to God. Music has a tremendous power to draw you to God. Get into the habit of always singing a tune. It will give you a new life and send joy into your soul. Then you will be able to bind yourself to God. It is especially good to sing on Shabbat and the festivals, and at a wedding celebration. (From Advice 'melody')
אז ישיר משה כו'. פי' אחר יצ"מ נעשו בנ"י כלים להעיד על הבורא. כמ"ש עם זו כו' תהלתי יספרו. ובמדרש ויעלני מבור שאון כו' ויתן בפי שיר חדש. פי' חדש שהוא לעולם בכח התחדשות. וא"י להיות נשכח מנפשות בנ"י. ולא לחנם קבעו לומר השירה בכל יום. וזו האמונה שהאמינו בנ"י כי הישועה הוא לדורות ...וזו השירה והדבקות הי' נטוע בנפשות בנ"י מעולם. אך קודם יצ"מ לא היו יכולין להוציאו מכח אל הפועל. ואחר יצ"מ נתגלה זו התשוקה...וכן בכל ש"ק יש גאולה וחירות לזה הנפש והרצון. לכן הוא זכר ליציאת מצרים.
Then Moses and the children of Israel sang. After the exodus they became instruments to give witness to the Creator. As it states, "the people I formed for Myself that they might declare my praise (Is. 43:21). The midrash quotes the verse, "God lifted me out of the gruesome pit, the slimy clay, and set my feet on a rock, steadied my legs. god put a new song into my mouth, a hymn to our God. (Ps. 40:3-4). The meaning of "new" is that it forever carries this power of renewal. It can never be forgotten by the souls of Israel. It was not a throwaway that [our sages] established that we should sing this song each day. Israel's faith [at the sea] was the this saving act would last for all generations...this song and the attachment to the Divine have been implanted in the Jewish soul forever. But until the exodus from Egypt they were not able to call it forth. Only after this was the longing for God revealed...thus on every Shabbat the soul and desire are set free. That is why Shabbat is "in remembrance of the exodus from Egypt" [and why we sing God's praises on that day]