Hi all, my wife's Pixel (formerly my Pixel) starting having this issue shortly after the Oreo upgrade which became progressively worse over time. Her microphone was NOT broken and neither was any of the other hardware in her phone. This was purely a software issue and I can tell you what fixed it for us ... Android P. I enrolled her phone in the Beta Program so I could go ahead and upgrade her to P. After a week, she has only had 1 problem with her mic that could have been an unrelated issue. We've had some bugs, but that's to be expected with a beta version. Under normal circumstances you should never enroll your primary phone in the Beta Program, but her phone had become unusable and we had nothing to lose. I will update if we run into trouble going forward, but so far so good. I would highly recommend enrolling in the beta and going up to Android P.