Sometimes solving the problem can be way simpler than anticipated, and therefore you must exclude the obvious problems, such as pocket lint in the holes at the bottom of the phone, where the microphone picks up the air pressure changes that the phone interprets as sound.
So, check the holes at the bottom, and you might save yourself a lot of work. I have had this help for one of my customer's phone before. If this helped you, great! If not - which it usually doesn't - don't worry.