I’ve had an issue with FaceTime for Mac OS X for a few weeks now. Every time I would FaceTime or video chat over iChat I would have audio issues. The people with whom I chatted sounded distorted, and they in turn would say I sounded as though I were under water. I’m using an external Apple FireWire iSight camera/microphone. I’m not sure if this issue is exclusive to my set up, but judging from all the posts in the Apple Support Forums, it looks to be like a fairly common problem.
I did some digging, and the first suggestion I found said that it might have to do with the Midi setting in the Audio Midi application. I checked my settings there, but found nothing out of the ordinary, and was unable to restore functionality.
Finally, I found another thread where a user suggested making sure the input volume for my microphone not be maxed-out in the Audio control panel in System Preferences. Indeed, it was maxed-out. After reducing the microphone volume to a reasonable level, the problem disappeared.