How to replace the mic

Based on the teardown, it seems like most likely the microphone is soldered directly to the motherboard. (Though the teardown doesn't show a good image of the board.) This practice is very common on these kind of phones.

I would recommend cleaning out the microphone hole on the bottom of the phone.

I see this a lot with my customers. We clean speakers, microphones, and charging ports all the time.

I would take a small pin and gently insert it but not too deep (most phones are designed in a way that if you insert something in the microphone hole it won't damage anything, though I am not sure on this phone) to break any dust free. Then blow it out with air. and see if that improves the microphone sound.