Why can't I get audio on FaceTime calls that come in from my iPhone

I have a cam attached and audio works perfectly on FaceTime (video) calls made or received. But when my iPhone rings the Mac and I accept the car (audio) the person on the receiving end cannot hear any audio. I't chosen the mic in the cam and I can see it working.

Needless to say, I'm stumped.

Thanks for any input.

Joe