Sometimes, a person must build ladders to climb to the heavens. A nigun (song) is one of these ladders, specifically when we sing after the joy of a mitzvah, with a heart broken open…
Music in Judaism is a way to express how we are feeling, whether it be through prayer or simply expressing our emotions.
(21) Then Moses held out his arm over the sea and יהוה drove back the sea with a strong east wind all that night, and turned the sea into dry ground. The waters were split, (22) and the Israelites went into the sea on dry ground, the waters forming a wall for them on their right and on their left. (23) The Egyptians came in pursuit after them into the sea, all of Pharaoh’s horses, chariots, and riders. (24) At the morning watch, יהוה looked down upon the Egyptian army from a pillar of fire and cloud, and threw the Egyptian army into panic. (25) [God] locked the wheels of their chariots so that they moved forward with difficulty. And the Egyptians said, “Let us flee from the Israelites, for יהוה is fighting for them against Egypt.” (26) Then יהוה said to Moses, “Hold out your arm over the sea, that the waters may come back upon the Egyptians and upon their chariots and upon their riders.” (27) Moses held out his arm over the sea, and at daybreak the sea returned to its normal state, and the Egyptians fled at its approach. But יהוה hurled the Egyptians into the sea. (28) The waters turned back and covered the chariots and the riders—Pharaoh’s entire army that followed them into the sea; not one of them remained. (29) But the Israelites had marched through the sea on dry ground, the waters forming a wall for them on their right and on their left.
"This daily recital is mentioned by Rav Saadya Gaon but not as an obligatory part of the service. Rambam calls it a “custom.”2Hilchos Tefillah 7:13. Subsequently adopted by all communities, reciting the Shira became an integral part of our daily prayers. Some hold that the Shira should be chanted aloud while standing. It should be sung with joy as at the time of the Exodus. “Sheloh” writes that reciting the Shira atones for sin, so that if one wishes to repent a serious sin, he should recite the Shira every day with great fervor. The Zohar says, “If one recites this song with elation and joy, he will be worthy to sing it in the next world. He will also be worthy to sing it when he greets the Messiah.”
(19) For the horses of Pharaoh, with his chariots and riders, went into the sea; and יהוה turned back on them the waters of the sea; but the Israelites marched on dry ground in the midst of the sea.
Jews were encountering such an incredible moment, that they simply could not speak about their feelings! They had to begin to sing to express how they're feeling.
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 man and he didn't say a song. God saved Abraham from the fiery 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 it says Then Moses sang and the people of Israel.
First time in Torah we see song, Jews had so much emotion that they couldn't contain themselves!
(20) Then Miriam the prophet, Aaron’s sister, picked up a hand-drum, and all the women went out after her in dance with hand-drums. (21) And Miriam chanted for them:
Sing to יהוה, for He has triumphed gloriously;
Horse and driver He has hurled into the sea.
Miriam was a naviah, and encouraged everyone (men and women) to burst into song. Was singing to Gd directly to express her joy.
Iyun Tefilla
Were there not other times that Israel believed in Gd? What happened here?
Faith has different strengths throughout different points of life. The stronger that faith is, the more miracles one can see every day.
Hilkhot Tefilla
One should say the Song at the Sea with great joy, and imagine that at the very moment he crossed through it and was liberated. And if one does so, then all of his sins will be forgiven. In other words, this is not just about reciting or retelling the event; it is about reliving it.
Rashi
Upon seeing this miracle, the Jews chose to sing. No one forced them- they saw this miracle, and knew that they had a decision to make- to sing or not to sing?

