Marg Mowczko notes the 'them' in Exodus 15:21 is masculine — Miriam is singing to men, not just women.

Mowczko points out that while Miriam leads the women out with tambourines, the 'them' she sings to in verse 21 is masculine — she is encouraging men to sing to the Lord. When you look at the full picture, it appears to be men and women joining in chorus together singing to God. There is no clean separation of Miriam speaking only to women.