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Sounds

• ​http://www.flashkit.com/ Huge resource for music loops and sound effects. (Attention!! Opensourceness is yet to be confirmed)

Music

• ​http://opsound.org/ Nice music under CC-by-sa-2.5
• ​http://commoncontent.org/catalog/audio/music/ most of them are NC, but some are free (by-sa or PD).
• ​http://www.newgrounds.com/audio A lot of music of any kind. (Attention!! Opensourceness is yet to be confirmed)