This climate has had a chilling effect on independent filmmakers hoping for a theatrical release. Many major label tracks are made available with a "festival license" option, meaning you can pay to use it on a version submitted to festivals, but if you plan distribution beyond that, you have to pony up big bucks. Luckily, this has led to many more independent filmmakers seeking out tracks from unsigned musicians for much more reasonable terms. At any rate, you should check out Killer of Sheep if you haven't. The songs really do make several of the scenes work, and you'll appreciate that Burnett stuck to his artistic vision, even if it took 30 years for most of us to be able to see and hear it.
Killer of Sheep: The Charles Burnett Collection (via Amazon)