How can I fix the phone's mic problem?

I have a Nexus S I got off of a friend, and have successfully replaced the earpiece, buzzer/loudspeaker & USB charging port. All that's left, because it's not working, is the mic. No one can hear me when I make calls. The rubber boot connected to the rectangular gold mic is missing. I'm not sure if it is necessary or not. I've replaced the mic with another, matching the four dots underneath and applied it in properly. Still not getting anything. I'm out of ideas. Any suggestions?...