In the Exodus narrative as we have it, the Song of the Sea (vv. 1-18) is followed by a brief summary notice (v. 19) of the miraculous event at the Sea—with its effect on the Egyptians and the people Israel, respectively. Then, in verses 20-21, it is recorded how Miriam led the women of Israel in singing and dancing to the Song. Only the first stanza (that is, the first two couplets) is preserved (cp. verse 1), but it is fair to assume that this is an abbreviated, shorthand representation of a longer song. The historical data in vv. 20-21 would appear to be authentic, given the cultural parallels in Judges 11:34 and 1 Samuel 18:6, recording instances of women singing and dancing in response to a military victory led by a hero of the people (Jephthah, David). Here, of course, the underlying idea of the Song is that YHWH, God Himself, is the warrior who achieved victory over the enemies of Israel, and without reliance upon any human intermediary or military technology. Thus, God is the conquering hero greeted by the women in celebration.
The way this material is presented in the book of Exodus has led some critical commentators to posit that we have two distinct lines of tradition preserved, regarding a song that was sung following the event at the Sea. In one line of tradition, the Song is attributed to Moses, in the other, it is to Miriam, who is declared to be a prophet (ayb!n`), like Moses, in her own right (v. 20). In some ways, it is more appropriate that she should be the one inspired to utter this Song. In addition to the example from 1 Sam 18:6ff (cited above), we have the great songs attributed to inspired women in Judges 5 (Deborah) and 1 Samuel 2:1-10 (Hannah), not to mention the oracular utterances in the Lukan Infancy narrative (Elizabeth/Mary, Lk 1:41-55), as well as other examples that could be cited. The parallel with Judges 5 is surely the most relevant, coming as it does after a great victory over the enemies of Israel, won in large measure by the intervention of YHWH, fighting on behalf of his people, utilizing the forces of nature (Judg 4:23; 5:4-5, 20-21). It is said that Barak sang the song, along with Deborah (v. 1), but surely it is Deborah who should be credited, according to tradition, as the inspired author of the song. Much in the same way, Miriam would have been inspired to utter the Song at the sea.
One interesting critical theory is that the older/original tradition attributed the Song of the Sea to Miriam, but that, as the narrative developed, this was shifted over to Moses, who was the more prominent and prestigious figure, and the one deemed more fitting to be associated with the Song itself. If the Song originally developed (and was transmitted) separately, it could have been included at either point in the narrative—that is, attributed to Moses, as the leader of the people, or identified with the tradition of the song uttered by Miriam (v. 21). Some would go so far as to claim that the Song itself came to be constructed on the basis of the two couplets of the “Song of Miriam”, as preserved in the early tradition, with the Song of the Sea being later in composition than the original couplets.
Returning to the actual text, as we have it, following the notice regarding Miriam and the women (v. 20), the couplets comes in verse 21. They are virtually identical with the opening couplets of the Song (cf. the note on stanza 1), the only difference being the form of the initial verb:
“(Come, and) sing to YHWH,
for raised, He (is to be) raised!
(The) horse and its ride [bkr]
He has hurled in(to) the Sea!”
The Song of the Sea in verse 1 begins hr*yv!a*, “I will sing…” or “let me sing…”, while here it is an imperative (Wryv!, “sing!”, “you must sing”). Some commentators would hold that the “Song of Miriam” here preserves the proper (original) verb form for the Song, which fits the terse 2+2 meter better that the imperfect form in v. 1 as we have it. The message of the Song in its opening couplet is the same in either case, giving praise to YHWH for His defeat of the Egyptians through the nature-miracle at the Sea; this miracle demonstrates His power (as Creator) over the entire natural world, but especially the forces of the sky (wind and storm, etc). It was this victory over the Egyptian threat that allowed His people to escape, and, as they ‘crossed over’ the Sea-region, Israel came to be formed as His people in a new way, a process of fulfilling (and re-established) the covenant bond that would find its culmination in the theophany at Sinai (chaps. 19-24) and their eventual settlement in the Promised Land.