Wired has released a CD with some open-source songs for users to: Rip. Sample. Mash. Share. with others online.
While the songs themselves aren't exactly "open-source" they do have Creative Commons "sampling" licenses which allow "ripping, sampling, mashing and sharing" with others.
Check out the link above for more information about participating artists and how the songs on the CD may be used. You can also check out the CC Mixter - The remix family tree site based on tracks with "mashes" from the CD.