iMac internal microphone not working in Facetime

The internal microphone of my iMac is not working in FaceTime.

In sound preferences, I have the internal mic set for input, and this is also true in Audio/MIDI setup.

In Facetime, I have Video>Microphone>Internal Microphone selected. There is no corresponding menu item for Audio (perhaps this is the problem?).

When I make a Facetime audio call, I can hear the recipient, but they cannot hear me. This is regardless of whether the call is to a landline or another FaceTime user.

When I make a call in Skype, no problems at all.... so the microphone itself cannot be faulty.

Anything else I can try?