My internal microphone does not appear to be working in my mid 2007 iMac using FaceTime.

I have a mid 2007 iMac that has been upgraded to El Capitan. When I try to use FaceTime, I can hear the caller but they cannot hear me. When I check the sound settings, the internal microphone is selected and the levels are normal. I do notice however, that the input levels do not move at all. Any ideas?