Grease
American Graffiti
Amadeus
Wayne's World
Fiddler on the Roof (even though I still have yet to watch the whole thing )
Seven Brides for Seven Brothers
Haven't seen them yet, but A Hard Day's Night and This is Spinal Tap look pretty awesome.
The Shawshank Redemption isn't about music specifically, but there is a part that shows the amazing power of music to transcend our surroundings- the main character, who has a life sentence at Shawshank Prison, plays an aria from Mozart's The Marriage of Figaro over the prison PA system, and for a moment every man there feels free. The main character has to do a week in solitary confinement for it, but says it was easy for him because he just listened to the music in his head. The score to that movie is also very good.