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.
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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.