This solution solution shows the components location of Nokia N97 microphone circuit that needs to be check if the problem occurs.
Nokia N97 Mouthpiece Microphone Solution
Ensure to restore or flash the device firmware first before trying to calibrate hardware components.
If flash firmware updating is already been done and nothing happens , you may then proceed to troubleshoot hardware components.
a) Trace and check the supply lines paths of the microphone module.
b) Check and or replace the resistors highlighted on solution above, the two resistors have 0 ohms resistance value while the other one has 100 ohms.
c) If all the resistors seems all working and or already replaced and the supply voltage is okay, you may then replace the digital microphone module.
For further troubleshooting Nokia N97 Microphone failure, there are instances that you may need to rework the audio IC(AVILMAS) and sometimes the problem lies on a faulty CPU IC (RAPIDO).