I had the same problem and found the solution. The quiet microphone problem after screen replacing happened to me because I forget to put a little adhesive (its a little black square seal with a circular hole) that creates a sort of “tunnel” between the microphone (that is on the motherboard) and the hole in the front panel. Without this seal the sould of voice does not reach directly the microphone and so the voice appears very quiet.
Look for Microphone Adhesive - 1301-7388 on google.
I moved the adhesive from the old screen and now the microphone works perfectly.