Q: imac - facetime - constant short-term (seconds) mic problems

hello there!

me and my wife regularly chat on facetime. but for some reason I can't understand, when she uses her imac, several times during any conversation her mic just goes dead. I can ser her lips move, but nothing comes out. I need to ask her to repeat over and over and over.

We've tested google's video chat embedded on gmail webpage, and her mic works perfectly.

has anyone ever faced such problem?

thanks in advance!