I found this list of the top 25 music films on a blog called coffee and celluloid. A lot of the films i have never heard of but they all sound interesting.

The page is chalk full of synopses and links.

http://coffeeandcelluloid.com/

here is the list itself:
1) Superstar: The Karen Carpenter Story (1987)
2) Lets Get Lost (1988)
3) Stop Making Sense (1984)
4) The Last Waltz (1979)
5) The Decline of Western Civilization (1981)
6) The Blues Brothers (1980)
7) Woodstock (1970)
8) The Wall (1982)
9) Hedwig and the Angry Inch (2001)
10) Heavy Metal Parking Lot (1986)
11) Coal Miner’s Daughter (1980)
12) 24-Hour Party People (2002)
13) Bird (1988)
14) Quadrophenia (1979)
15) Rocky Horror Picture Show (1975)
16) Performance (1970)
17) Pump up the Volume (1990)
18) What we do is Secret (2007)
19) Sympathy for the Devil (1968)
20) Kill Your Idols (2004)
21) Velvet Goldmine (1998)
22) Wild Zero (2000)
23) Heavy Metal (1981)
24) Flash Gorgon (1980)
25) Dogs in Space (1987)

Enjoy.