Also had similar problems, tried a few things at once and I got the mic working:
The grounding cable. From the bottom left corner under the headphone jack to above the loudspeaker, on the opposite corner of the headphone jack. Make sure it's seated correctly. This could prevent the mic from working.
The washers. I think I might have had the loudspeaker bottom-right corner washer placed incorrectly. It fell off when removing the loudspeaker.
There also was some sort of rubber/glue type of stuff on the mic - compared to the old mic, it was almost twice as thick, so I removed the sticky stuff (probably just double-sided tape) just to be sure (I now see a clear hole in the middle of the new part). The rubber housing for the mic should also be installed correctly for clear sound.
Having fixed these things, I got mine working. Maybe test the old mic again, if that one works take a close look at the new part's mic. If all else fails, probably a faulty part. Good luck guys. Oh and also, don't bother removing the board for this repair - takes you at least twice as long and more risky.