My iChat routinely loses sound along with mail. Usually after waking from sleep when they are still running. This might not be the same problem you're having, but what I've found that works when iChat sound goes away (other sounds don't) is to quit iChat (and mail) launch activity monitor, then quit the process coreaudiod. You'll need to enter the admin password to quit that process. The OS then relaunches that process and iChat sounds are back.