Opsound - Sounds and music under "Creative Commons Share Alike license".
CCmixter.org - Music under Creative Commons Licenses. Important: Only music under Attribution can be used. Note: Be careful some users may not license their work properly, because they remix work under other non compatible licenses or they may play copyrighted work. Creative Commons sampling licenses are not free enough to upload to Wikimedia Commons.
Raita is a collection of digitized early Finnish sound recordings provided by the Library of the National Library of Finland. All music files are under public domain (over 50 year old recordings of public domain music).
The Mutopia Project. Free sheet music for everyone. "Most of our music is distributed under Creative Commons licenses. Each piece clearly lists what license it is distributed under."
Freesound.org. Collaborative database of Creative Commons licensed sounds. Freesound focuses only on sound, not songs.