I have an audio podcast and often use the Podsafe Music Network. music.podshow.com . I had a recent project to post a web-only video. I asked for–and got — written permission from the composer to use a piece in return for credit on the video. Many of these composers looking for exposure will charge little or no fee. Also, if your project is non-profit, I stumbled on a website by the composer Moby, to obtain a limited license for some of his music for free.
