Definitely use to tear down above to disassemble your device and look for where the mic is to see if there is any clogs of dirt or possible moisture damage. If you see any white crustiness any where on the board, go ahead and grab a tooth brush or something similar to it, and use a little bit rubbing alcohol to clean up any areas that may have been exposed to any liquid damage. If that still doesnt work you will need to replace the mic which is soldered to the board and is not an easy job if you have never done micro soldering before.
Hoped that helped, have a good day.