The YouTube Blog today announced a couple of new features, including this "AudioSwap" from the "Test Tube". (YouTube's Google Labs)

The blog explains it as "a new feature that enables you to replace the song on your video with music from a library of free tracks donated by our music partners". – who are the partners?

links:
youtube.com/blog?entry=JsiM84a ...
youtube.com/audioswap_main
sketchswap.com