I finally got her to sit down at her iMac and try doing what you suggested. You nailed it.

Apparently Macintosh doesn't follow the common tradition of clicking the little triangle next to the icon to bring up the sub-menu. That was unfamiliar to her. She just clicked the mute icon which, of course, did nothing useful. Once I showed her how to bring up the sub-menu, then it was a simple matter to choose the correct microphone.

All is right with the world now. Thank you, Mark.

tanstaafl.