I think of the flawed early 70's Michelangelo Antonioni film (which I love, for the record) Zabriskie Point as a film that was greatly improved by its soundtrack - including Pink Floyd, Jerry Garcia, and numerous others. Interesting film, considered a bit of an embarrassment when released for the golden boy of Italian cinema (Blow Up is another touchstone sixties film of his with a great soundtrack), but I've always thought it worked very well, and I can't help but think its reputation will revive over coming generations.