I found this interesting post on a similar mic issue thread for the iPhone X (thanks to brentwoodkennyg):
I sound muffled or underwater, but only sometimes. Other calls are perfect. Tried all of the fixes in this forum - Network reset, LTE off/on, full reset, wi-fi calls off. Noise cancellation off. With and without cover. Got a replacement phone from the Apple store. None of it helped and now I know why...
The issue turned out to be how I was holding the phone. There are three microphones on the X and one of them is located by the camera on the back of the phone. Even though I didn't cover it while holding the X (with fingers and thumb on the sides of the phone, I apparently was close enough to make that mic behave differently. When I move my phone-holding position in the same grip to the bottom part of the phone, the problems vanish. I have run through a series of test with my "complainers" and this seems to pinpoint the root cause. It's either user error or a design flaw, but there is a simple workaround. Hold the phone at the bottom of the case and definitely not near the top. Problem Solved!
Update as to why this works: Apple has designed noise cancellation into the talking side of the phone. (This is different than the noise cancellation in the Accessibility settings - those are for the listening not speaking). Anyway, there is a small mic on the back of the X by the camera. It's purpose is to listen for ambient noise (think crowded bar) and to cancel that noise from what it hears from the mics on the front at bottom of the phone. Those mics hear your voice talking plus they hear the ambient noise (crowded bar). The software "subtracts" the bar sound from what it hears on the front and the listener never hears the bar noise - just your voice. (You can test this by completely covering the back mic hole with your finger - then the listener hears the bar noise and has trouble hearing your voice).
Anyway, this is a really great feature - except that if you hold the phone near the top, it channels part of your voice into that back mic and it ends up canceling out all or part of your voice. So I have to hold my phone at the bottom and things work fine. This also explains problems with folding phone cases and issues when you are holding the phone between neck and shoulder.
