Have you tried recording a voice memo? If you record, a voice memo make sure the little speaker symbol in the top right is white (turned off) when testing the earpiece microphone. This means the Phone will only record using the earpiece microphone. If you can hear yourself clearly, you know it's a software issue. You can do the same with the speaker symbol turned on so that it records using both microphones. If the audio is bad, you know it's a hardware problem.
Hope that you found this usefull, and let me know the results. I might be able to help out
Justus Darley
www.jfixit.nl