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.
Nokia C6 MIC, Mouthpiece Microphone problem solution
Try to check the Mic data signal connection line paths including the microphone's supply voltage, before replacing the digital microphone module.