How to Replace the Mic?

So I bought a iPhone 4 with a cracked screen and a mic problem. I am able to replace the screen, thanks to this site ;). But the previous owner said that there is a mic problem, where the other person cannot hear you. Everything else works perfectly fine on the phone. Also im not using the phone with the network its on, so cant check if its working. OR do I just open the Voice Recorder and see if it records my voice?

How do I know where the mic is (and how it looks like, so I can get one) and how do I go about replacing it?

Thanks in advance guys!