When the customer threads over on the Google Support Forum start to get hundreds of replies deep, you know there's a serious problem afoot. This one is affecting the Pixel and Pixel XL, and apparently occurring to a large enough subset of owners that Google is taking its response more seriously than the usual bug. Owners are reporting complete failure of one or more of their microphones, sometimes causing a total audio input block, sometimes occasionally working with the camera app while recording video.
The good news is that the Google employees in the gigantic support thread are being, well, supportive, with a lot of good information on what's probably happening. According to a lengthy breakdown (no pun intended) by Google employee Brian Rakowski, both the Pixel and Pixel XL may have a failure-causing physical default in one of the three microphones on the devices, the primary "phone" input and two ambient mics. It could also be an issue with a hairline crack in the solder of the microphone component going to the phone's main board. These two issues seem to cover most of the device problems experienced so far, and the minute physical nature of the defects explains why they can sometimes appear and disappear with tiny variations like phone grip and ambient temperature. Some users have managed to get their microphones "un-stuck" with applied heat and air via a hairdryer, but it isn't officially recommended. The problem is occurring with "less than one percent" of units.
The bad news is that, since these problems are entirely hardware-based, there's no easy fix. Someone with some basic soldering skills might be able to crack open their phone and fix one of the issues... but there's no reliable way to make sure that actually is the problem, and there you are with a possibly broken phone and a voided warranty. Nope, the best solution at this point is to get a replacement from Google, since all Pixel and Pixel XL devices out in the wild should still be well within their warranty period. Unfortunately, and as confirmed by the Google support staff in the thread, there's no guarantee that your replacement phone won't have the same problem.
- Google Support Forum