Facetime sound from iMac (Yosemite) isn't working; video is.

Newbie to Facetime. Tried to make a video call from my 2008 iMac (Yosemite) to my wife's MB Air (Yosemite) an ocean away and the video worked perfectly and I could hear her, but she could hear me only very faintly or not at all. Then I used the iMac to video-call my iPad 2 (iOS 8.4.1) in the same room > perfect video, iMac received audio, but only very, very faint audio out of the iPad except for the ring tone of the call. So the problem is definitely with the iMac. Opened Sound pref, made sure the internal mike was the selected input method, and tested the internal mike (the input level registered to the fourth bar with normal speech volume level).

Anyone have any ideas how to get the audio working?