20 And Miriam the prophetess, the sister of Aaron, took a timbrel in her hand; and all the women went out after her with timbrels and with dances.

21 And Miriam answered them, Sing ye to the Lord, for he hath triumphed gloriously; the horse and his rider hath he thrown into the sea.

We have not heard of Miriam since she followed baby Moses floating down the river as a baby (the common assumption being that Miriam was the unnamed sister in that story). Now we hear that she had grown to be a prophetess in her own right, and apparently a leader of women. At her example, all of the women joined in music and song and dance. I would assume the song that they sang was the same one we were given over the last nineteen verses.

I find something very appropriate in it being the voices of women that sang praises for the miraculous avoidance of war. War is antithetical to the female instinct to put down roots and build a home. War undoes the miracle of life that emerges from the body of the woman. War and womanhood are completely at odds to one another, so it is very fitting that it was the voices of women praising that war, itself, had been defeated that day.