Try OpenGameArt if you need sound effects.
There are other free games "inspired" by some original game that started with art and sound from that original, but have created free replacements.
E.g. OpenTTD can use sound from the original TTD-CD, but has OpenSFX
as a free replacement for sounds.
So, look around what other free games have done, e.g. Ufo2000, UFO:AI, FreeOrion etc.
Also, see FreeSounds.