did you have video conference while you were on ichat?... I 'm asking because that has happend to me twice. I dont' know if it was just a small glitch but here's what i did.
I quite ichat and restart it, then i am able to hear the IM's..
BUT when i video conference then the IM's aren't hearable anymore.. Then i finished video chat, closed (NOT quit) the IM's and all the incoming IM's were good again... Since then i haven't had video conference but all the sounds come up without a problem.
If that doesn't work, try shuting down your computer and restarting. I would because i usually put my comptuer to sleep everyday for a week or 2.
Last case senario, check for permissions errors. but that should but all.