To fix MIC problem on Nokia 1680 and 1681 classic, you may first check the microphone module using a multimeter - to confirm if it's still working or not. You may also clean the MIC's terminal contact pads if there's an oxidation build up found.
If the microphone seems okay and the contact pads is cleaned but the problem still exists, refer to the solution below and check the connected components to it and trace also its connection line paths.
Nokia 1680c and 1681c MIC Connection Line Paths
You may check and replace all the corresponding filter resistors and capacitors. If all the resistors had been check and or already been replaced but doesn't fix the problem, check the MIC Bias Voltage, if the voltage is missing,. The possible faulty is the UEM IC, reworking it might only fix the problem.