This worked for me a few months ago - my mike would be inaudible, except for keystrokes, tapping on the body of the computer, like someone mentioned earlier in the thread. I futzed with ambient noise reduction, everything else in settings, no change.
It's a little gross, but there's nothing wrong with your mike - the holes on the side of the air are drilled so small they get easily and thoroughly clogged with everyday crud. you need something extremely thin, like a single strand of speaker wire - then gently, and without pushing in too far, clean out the microphone holes. It'll work as new.
Great! This works. Mic is working perfectly again.
Thanks for the help.