Use music when talking about music in general, or a type/category of music:
▶️ My neighbors are playing loud music; it’s really annoying!
▶️ I’m not a fan of country music.
▶️ The performers are dancing to classical music.
Use song when talking about one specific piece of music:
▶️ I love the song “Imagine” by John Lennon.
▶️ I like two or three of the songs on this CD, but the rest aren’t that great.
the musics on this CD
▶️ This is the #1 love song of the summer.