Just a quick mention that it seems there are different problems being reported here, which may or may not have the same cause. To avoid confusion, I’m listing the most common ones (but unfortunately there do not seem to easy fixes reported on the forum for any of them).
- Microphone works in the beginning of a call, and then stops picking up any sound:
This topic. In most cases probably related to the proximity sensor. - Normal microphone doesn’t work in calls, but hands-free/conference calls are OK:
Also discussed here.
Note that if you also notice that audio recorder doesn’t pick anything up, but video recorders do pick up sound (as described here), there is also a possibility that there is a hardware problem in the bottom module. (Speaker mode and video recording use the secondary microphone, which is in a different module). - Microphone picks up sound, but audio quality is poor:
Also discussed here.