If encounterd and try to fix this problem, refer to this solution below an locate each component of the microphone circuit on the PCB board layout.
Nokia 6720c MIC Problem Solution
There are four DIGI MIC or digital microphone in nokia 6720c circuit, the two were shown on the solution above is for the voice call microphone.
You may first check the digi mic supply voltage line paths.
Then try to replace the microphone if the supply voltage were okay.
There are also possibilities that the problem may lies on the power management IC. By reheating or reworking it may only fix the problem.