From the Magnatune web site:

What is "Open Music" ?

Open Music is music that is shareable, available in "source code" form, allows derivative works and is free of cost for non-commercial use. It is the concept of "open source" computer software applied to music.

All our music is available under the "Attribution-NonCommercial-ShareAlike " license from Creative Commons to promote these goals. Specifically, this means:

• You can listen to our Internet radio stations, download our free music, and share with anyone you like.
• Derivative works (for example: remixes, cover songs, sampling) are explicitly allowed. Some of our artists publish the "source code" to their music so you can rework and improve it. This includes scores, lyrics, MIDI files, samples and track-by-track audio files. If you make a great new version of our music, we'd love to know so that we can promote it!
• Non-commercial use of our music and its "source code" is free. However, if you make money ("commercial use") with our music, you'll have to "share the wealth" and give us and our artists a share.