Horrible high pitched noise from iSight mic only under OSX
It's quite old question but I had same problem with two USB cameras just few days ago. To fix it go to the MIDI setup (i.e. by Spotlight Search Bar). On the left side choose your USB microphone (USB camera) and on the right try to change frequency to 44 100Hz and format 16-bit. Previously I had 24-bit which causes the problem.