Display mic will not work with FaceTime

I have a mac Pro (the metal case older model) connected to a 27" Apple Display (pre-thunderbolt). When attempting to use FaceTime person on the other end can not hear my audio. When I go System Preferences-Sound, it shows that my input is Display Audio, and the input level meter is active showing that sound is being picked up by the microphone. I have looked at FaceTime preferences and any other place I can think of, but can not find a fix. I know that the mic is actually working for it works with other apps. What am I missing? Thanks!