I have had several water proof phones, and that typically happens when the phone have gotten wet and then dried. It seems that with a bit of bad luck, the mic

I have had several water proof phones, and that typically happens when the phone have gotten wet and then dried. It seems that with a bit of bad luck, the mic can get clogged up if a drop of water ends up in it. Try to hold the phone in running water so the mic gets wet, and then carefully blow out the remaining drop of water.