The internal microphone on a MBP is located inside the speaker grill, so I would check to see if any dirt or dust is covering or clogging the left hand (left side) grill where the mic is. If this is the case, attempt to clean it out yourself or get Apple to check it out.
If there doesn't appear to be anything covering the microphone up, try calibrating it through System Preferences -> Sound -> Input -> Internal Microphone. Here you can check the input volume, make sure your mic is working properly, and adjust ambient noise settings.
Good luck!