some things I didn't see in the thread:
Burt Bacharach's score for Butch Cassidy and the Sundance Kid is fantastic, especially South American Getaway. Mark Knopfler's score for Local Hero is great. And Quadrophenia, which was written from the record, is pretty damn good.
For movies with a musician theme, and leaving aside documentaries about bands, I'd have to suggest Spinal Tap, The Great Rock and Roll Swindle, The Commitments, Yellow Submarine, and maybe also Help!
For outright musicals, where people just start singing instead of talking, I'd look at Three Penny Opera, Disney's The Jungle Book, Guys and Dolls, Paint Your Wagon, Fiddler on the Roof, and Singin' in the Rain.
For singles out of movies, the definitive answer is rock Around the Clock from Blackboard Jungle. But check out Elvis Costello and Brian Eno, "My Dark Life" from the X-Files Movie. Phenomenal. Also, Cat Stevens' songs for Harold and Maude are excellent. Elvis Costello did a song for the Rugrats movie, with Gwen Stefani singing, called "I throw My Toys Around." It is nearly perfect. And Randy Newman's I Will Go Sailing No More, from James and the Giant Peach, is great. And the Portuguese versions of David Bowie's glam years in The Life Aquatic with Steve Zissou are priceless.
What a great topic. I will be up all night now.