One Laptop Per Child (OLPC) project are available for everyone to download, use and re-mix etc, from the sound samples page of the OLPC wiki (via the Creative Commons blog). (On the OLPC itself see this OLPC video.)
There are over 6,500 samples (over 8.5GB) available currently, including sound effects, background sounds or "soundscapes" whether from nature or cities like rain, trains, wolf whistles etc, loops, grooves, drum sounds, voices and all manner of musical instruments - even animal noises like frogs (& humans imitating animal sounds)!
The audio files are made available under a CC-BY licence - which means you can use them freely as long as you credit the author/creator in whatever way they say they want to be credited (e.g. "drum loop by X"). Each sample is 16-bit, WAV, Mono, normalized to -3dB, and provided at 3 sample rates - 44.1K, 22.5K and 16K. (You can convert WAV to MP3 using free open source software like Audacity.)
Another source of free audio clips I like is Freesound, which calls itself "a collaborative database of Creative Commons licensed sounds" - again it's sound effects and sound samples rather than songs. Their samples are available under a different type of CC licence, the Creative Commons Sampling Plus License (must credit the creator, non-commercial re-use, basically). I used a Freesound sample for the (real) Formula One racing car sound at the end of my BarCampLondon3 Segway video that plays as Jason zooms off on the Segway!