The next time you experience this problem,
1) Make sure the iPhone isn't in silent/vibrate mode (toggle switch on upper left side of the phone, above the volume buttons).
2) Adjust the volume using the iPhone volume buttons. If the volume display says "ringer" then no audio data is being sent to your phone. If it doesn't say "ringer", then audio data is being sent, but it's very low or mute of some reason.
3) Go to System Preferences -> Sound -> Input. Does the Input Level display show activity when you make noise close to your computer?
It's possible another application may have taken control of the audio input. Please quit any audio capture programs, such as iChat or Skype, to see if the problem goes away the next time you run iCamSource.
Is your computer going to sleep? It's possible that the program is unable to access the audio input device immediately after waking from sleep.