Well, there's http://www.freesound.org.
I'm not sure about the legalese on that site though. I assume this game will be free (as in open-source, not 0$), so it should qualify as non-commercial use. Then again, it is something organized, so some lawyers may well prove there is a touch of commercial.
Is there a lawyer in the house?
(also, i might be able to help a bit with sound editing should it be required; i have no musical sense or anything of the sort, but i can use the software )