I don't really buy the noise cancelation mic issue here, I am a iphone tech at the ifix repair shop, and I have replaced the bottom mic(contained on the click and charge jack) and the top mic, and in most of the cases it fixes the problem for a short period of time before it just comes back and the mic no worky!! Sounds just like whats happening here, turn a screw here and there and the problem seems to be fixed, but then returns, all false positives. That audio IC has to be removed and re-balled and resoldered back onto the board for this problem to go away, so far I have only done 1 wich was successfull. keeping in mind I was re-balling by hand 0.2mm solder balls with a needle, hotair station and a magnifying glass. very intense work, takes about 2-3 hours.
So to what I know this is really the only fix, keeping in mind that sometimes when you "reset all settings" on the phone it can also make the problem go away for a little while, so it's no surprise that turning some screws here and there will also yield such an effect or even replacing the one of the mic's, but the real and only culprit here is the audio codec people!!!