I just got my iPhone replaced yesterday because of a microphone which wasn't functioning. Weird thing is though, it would work when I was talking to someone on a call, but when I put an incoming call in Speaker Phone mode -- the incoming caller couldn't hear me. Also, the Facetime audio (on the other end) wasn't working either (e.g., the person was yelling into my iPhone screen "I can't hear ya, bud!")
Apple said, since it was partially working, that it must be a software glitch and told me to do a full software/factory reset, but it was to no avail. Argh, don't ask how long that took.
So they finally swapped it out.
Anyway -- back to your question: Not sure how widespread it is. Just hope I don't get a "stuck lock button" now.
Because the OP asked "Is this a widespread problem?" regarding microphone issues.
Then again, a fresh thread regarding "iPhone 5 speculation" or "When's the next White iPhone arriving??" might be better suited (err, 'warranted') for some folks.
Eh, this is a public forum. Ideas/topics/threads/banter get thrown about at will. I appreciated the opportunity to chime in.