[gr8belief]

Very perplexing software issue. I've wiped this 9900 about 8 times in the last 5 days, loaded current and previous OS versions. The fix works for an hour or two, then the microphone stops working again. It is out of warranty. I have spent several hours on the phone with the friendly people at BB tech support. I know this was a big issue a couple of years ago for ATT 9900 phones. This is a TMO, no damage, not wet, no funky apps ever installed, media card wiped and removed. I don't reload address book or other items, or sign in to BB ID. Anything I do besides dial or answer the phone makes the microphone stop working - sounds like I'm under water. But it will record video and voice notes with clarity.
I see lots of questions about this, but not a lot of clear answers. Anyone have any help for me? thanks!!!

[hamsterwheel]

That is strange. Seems you've tried everything. Might just be a faulty mic?

[pkcable]

I don't think the mic is the problem, but rather the noise cancellation hardware might be faulty. FIRST make sure the hole or vent for it is not block or dirty, and I'm not 100% sure where it is on the 9900, BUT just lightly blow out all the holes and vents with compressed air on it's lowest setting or with your mouth, or better yet VERY carefully use a toothpick! Secondly get and run this program....

BlackBerry Virtual Expert - BlackBerry World

It's a program from BlackBerry that can detect hardware failures, and if it detects something then the device must be repaired or replaced.