I had this problem two days ago. Microphone was not getting any input and the other end of the phone call couldn't hear anything. Pressing the phone's numbers did send DTFM-tones over though.

After I rebooted, mic was working again.

Then it happened second time yesterday. I noticed VUmeter didn't get any input from microphone. Also myDicto recorded only silence. Rebooting fixed the system again and I got mic working.

Because rebooting fixes the problem, could this be some kind of software bug? I haven't had problems before though and I try to remember what applications I have installed during this week when the problems started. QRadio is the only one I remember, but it shouldn't have anything to do with a mic, although pulseaudio issues come to my mind.

Any ideas and have others noticed microphone going dead sometimes and rebooting to fix it?

Edit:
Well, that was it. I started Qradio (without having anything blugged as a antenna), quit Qradio. After this micrphone is dead.