MBP 15" audio input very bad quality
My checklist would consist of ensuring that the mic is not covered by dust/particles, try to use audio-recording software to make sure it isn't a problem with your voice chat software, and use an external mic to make sure it isn't a problem with the system itself.  One issue with the 15" MBP 2011 is that the mic is actually near the left speaker grille, so if you're running more intensive software that will run your fan up, you may have issues with fan noise being audible on the voice chat.  Typing will also result in noise on the mic, moreso on the left side of the keyboard.  It's a specific design issue with the 2011 MBP 15", and is noted by users on numerous Apple forums.  Making sure that the mic/speaker grilles are clean can help with the audio clarity; however, typing will always be audible on the mic.