I've been having the same problem on Mountain Lion 10.8.2 recently,
Procaster was working perfectly for a few days till one day it just stopped.
I've tried two different USB audio interfaces now and the speaker audio only produces a quiet hiss on the stream, my logitech webcam gets it's audio picked up perfectly.
I did everything in ernielive's second post only to have it do the same thing once i reinstalled.
Any help guys?