After the collapse of the walls of water onto the pursuing Egyptians, Moshe and the people burst into song. How were several million people able to sing together? One famous Jewish source (Tosefta Sota 6:2) says it happened through a low grade form of prophecy: "The holy spirit came over them, and they praised Him."
Other ancient interpreters, pointing to the first words of the Song (I will sing unto God...") had other suggestions:
(1) Moshe sang, and those who heard him (ie not everyone) joined in for two choruses [Philo]
(2) Moshe composed it himself (and presumably taught it to the people?) [Josephus]
(3) Moshe sang it and the people repeated, line by line [Rabbi Akiva in m. Sotah 5:4]
(4) Moshe sang in sections, the elders answered " אָשִׁירָה לַיהוָה כִּי-גָאֹה גָּאָה סוּס וְרֹכְבוֹ רָמָה בַיָּם." and the people answered עָזִּי וְזִמְרָת יָהּ, וַיְהִי-לִי לִישׁוּעָה [Tibat Marqa]
It should be noted that the LXX and the Samaritan Bible both have SING (plural) rather than I WILL SING.
Additionally, there are traditions that everyone at the Sea experienced prophecy. This is concluded in two different ways. First the song itself seems to indicate that the people saw God. After all they sang "This is my God and I will glorify him." The word "this" seems to suggest they saw something. [Mechilta De Rabbi Yishmael] Meanwhile, in Deut 26:8 Moshe says "the Lord brought us out with great fear." Where the MT has "fear", the LXX and Vulgate have "visions" and Targum Onkelos, Targum Yonatan, and the Peshitta have "sights."
In the Pesach Haggadah, the gap between the two readings is bridged with the drash that the great fear was caused by the revealing of God's being, which would have also been the great vision/sight indicated in the other sources.
[Based heavily on James Kugel]
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