I've had three iPhones - the Edge, and two 3GSes. Each has suddenly and inexplicably had the microphone quit working.
The old silverbacked Edge was about 18 months old when I started playing the "Hello Game" with callers. It's played something like this:
Me: HELLO. CAN YOU HEAR ME?
Me: I'M HERE! HELLO. CAN YOU HEAR ME?!!
... aaaand scene.
I was hard up for cash at the time, so I limped it along with the earbuds constantly tangled in my pocket until I could afford a 3GS. Two weeks later I was playing the "Hello Game" again and back at the Apple store exchanging the 3GS.
The second 3GS just passed the 18-month mark. Last Wednesday I was making phonecalls. Then Thursday, nobody can hear me.
I have no idea what is causing the microphones to just drop out, but it's obviously a real issue. I conducted a few tests with the voice recorder and found that if I screamed directly into the microphone, I could faintly hear myself during playback.
My first step was to restore the phone just to make sure it wasn't a software problem. No improvement.
Step two was to vacuum the dock connector end of the phone. I figured that would pull any lint out and maybe dislodge lint that might have worked its way inside the phone. No help.
My last try before throwing in the towel and calling one of the questionable iPhone repair persons on Craigslist (I'm doing all I can to avoid buying the iPhone 4 and extending my AT&T contract another 2 years) was to spray a few bursts of CRC electronics cleaner up throught the microphone grill. I made a test recording -- slight improvement. I gave the mic another few bursts with the spray and then left it to dry for a few hours.
VOILA! My whispering iPhone was healed! The answer to, "Can you hear me now?" was a loud affirmative.
Now the downside. I've sprayed CRC electronics cleaner into my phone before. When you do this, it gets behind the screen, and you see bright spots where it's wet. However, last time I left my phone off for an entire day and they had all disappeared by the time I turned it back on. This time, I kept my phone on. (I know, I shouldn't have, but my sister's having a baby any minute, and I've got to be able to take the random crazy calls from my mother.)A full two days after spraying the cleaner into the phone, I've still got strange bright spots on the screen.
I have no idea if these are now permanent or if they'll go away with time. Either way I can live with it until either the iPhone 5 blows my mind enough to upgrade or I switch to Sprint and save $40 a month.