This solution may helpful to fix Nokia C6 microphone or mic mouthpiece problem. This problem occurs if the device flash firmware is corrupted or due to hardware failure.
Updating the device flash firmware may possibly help fix and solve the problem, but if it doesn't work
you may need to proceed into hardware repair and check certain components that holds for Nokia C6 microphone to work.

The solution below shows the location of the digital microphone module in Nokia C6 PCB board.
You may need to check or replace this components when found damaged or faulty.