Try the diagnostics menu under About phone and check for microphone test. If the results are fine, then no problem. If the occurrences are rare, then it could be specific network related issue also, which you shouldn't worry and leave the device as its at its best.
If you still feel, you want to try experimenting, then
try doing a software repair. But don't forget to take backup prior to doing repair as the entire data would be wiped out.