FreeDriod is a GPL licensed game with a collection of sound-effects. Some of them might be useful in-game.
See
http://freedroid.cvs.sourceforge.net/freedroid/FreeDroid/sound/effects/I have talked to the guys at #freedroid on freenode (IRC) and they would be okay with us using some of them (and they are also under the impression they are GPL licensed like the game itself). However we might have to clarify this situation further just in case.
Does anyone know of other potential sources (under GPL or an unrestrictive CC)?