High Matt,
You need to explain what mic? Bottom or front facing?
Because there are 3 mics in your phone.
One is at the back of the phone, small hole next to the back cam, used for "noise reduction."
Second (main) is at the bottom, used for placing normal calls.
Third is next to the front facing cam, used for placing hands-free calls.
This third one is on a flat cable that also comprises front facing cam, proximity and light sensors, and of course ear speaker.
Often during screen replacement, that flat cable is not properly reseated, or plain damaged.
Often also, the front facing mic is not properly inserted in the rubber gasket.