ccMixer is community web site which allows its users to create some 'new wine in old bottles' i.e. create new music from already existing music. All music is available under the Creative Commons license, which is nice. There should be more sites like these for other information or data.
It allows search for music, but that's not the focus. Searching the web site appears to be a simple string pattern match, and it really doesn't work that well from what I can see.
If you are an artist, then there are a number of ways to search for music largely by browsing (i.e. tag cloud).
(h/t: Lawrence Lessig)