Charles Ellicott Commentary Exodus 15:21

"And Miriam answered them, Sing ye to Jehovah, for he hath triumphed gloriously; The horse and his rider hath he thrown into the sea." — Exodus 15:21 (ASV)

Miriam answered them. — Miriam and her maidens at the close of each portion of the "Song"—i.e., at the end of Exodus 15:5; Exodus 15:10; Exodus 15:12; Exodus 15:18—sang the refrain which is here given—a refrain very slightly altered from the opening verse of the "Song" itself, marking, no doubt, the time with their timbrels, and moving gracefully through a stately and solemn dance.