My wife's iPhone 4's microphone stopped working. She could receive phone calls but could not be heard. This was an intermittent problem that turned into an always problem. Videos on you tube were helpful in showing just how to place a folded business card over the microphone.
The screwdriver worked perfectly to remove the screws necessary to get the back off. This is a very easy fix for a broken microphone.
You will have to use items of varying thickness in order to get the phone to work. Leave yourself a voicemail to see if the microphone is working again. If it works the first time but doesn't work later, your item is not thick enough. Just try it again and add some thickness until it works. You'll eventually get it. Good luck!