I have 3 solutions
this works on Iphone 4
i have tested all 3 of them, and have had success with all 3 solutions..
Iphone 4 lower microphone stops working.
recording video and memos works fine, but people cant hear me during calls.
just to clear things up for everyone...
when using video recording, memos or anthing else that can record your voice, the iphone is using the TOP MICROPHONE.
when calling/getting calls the iphone will be using the LOWER MICROPHONE.
When you put it on "Speaker" then the phone will use both.
Change the Dock Connector
if that wont work... then....
Click on this link, and follow the instructions, on how to fix the Compiler Chip.
iPhone audio problem: fixing an audio compiler chip fault
sounds and looks wierd...., but yes it works....
done it with multiple phones so far...
if solution 1 does not fix the problem, then solution 2 should help allot.....
should the problem still persist, then you might have a problem with your Home button.
See Solution 3 for more info
Change the Home button.
this last solution was the problem solver for me... i had 7 dock connecters, new and from other broken phones i tested with.
in the end, the home button just gave up when i was swapping around, then i just replaced it, and Voila.... my microphone was working again, and my home button was working too :D
i cant really explain what really happend, but its my 4th iphone i have had these problems with, and all has been repaired with these 3 simple solutions...
i really hope it can help alot of people in here, with issues with iphone 4 microphones not working