From here: http://www.macosxhints.com/article.php?story=20041230144746554 this could be helpful:

It turns out after some poking around on the Apple Discussions site, that this setting now lives in the Audio MIDI Setup app in the Utilities folder. Starting this up, there are three "Thru" checkboxes next to the microphone settings at the bottom in the Audio Devices pane. Even though none of these were checked, clicking the Master one for the mic on and off fixed the problem for me on both machines.