Let be start by saying my wife's iphone4 is now working :o)
.. not from Apple but from a noddy fix on the Internet!
I had visited the Apple Genius twice (Bluewater, UK), each time they looked at the phone and saying it is out of warranty but they will have a look.
Apple diagnostic looks okay, clean the 'mic' at the top, but no go. Suggest I reset to phone as new, update to latest OS, etc. etc.
As I have done that already, all they can do is suggest I paid £120 for a service call and they can replace the phone.
I explained there are loads of people with the problem, even on the Apple forum.
His comment was, if it were a problem, you would have people queuing in the store!
.. my comment to him was I'm not paying £120 for an Apple design/manufacturing fault and not for a phone £500 phone just over a year old!
Anyway, from the Internet, I have found there could be multiple problems with the mic (phone app) not working
1) mic at the top
clean out with a pin - Apple Genius is aware of this
2) mic failure, required replacing of component - engineer is required
3) intermittent and/or crackly on phone call .. this is mine
pressure to a part of the phone will fix this
to do this, you need to,
- un-screw the bottom 2 screws (mine is a Philip PH00 - cost me £1 )
- slide the back cover open
- place a 'L' shape (eg. business card), if too thick you will not be able to close the back cover
(to test, whilst back is open, call from another phone and press around the 'L' shape area, you should be able to hear when the correct bit is pressed!)
- slide the back cover back
test again before replacing the screws
The genius is this guys,
http://www.youtube.com/watch?v=EZmSVt8bMMo
this is the clearest video,
http://www.youtube.com/watch?v=0jhWxYTVxe4&feature=related
Good luck.
PS. I will think twice before buying from Apple in future!!!