Well, this sucks.
Android Police is some Pixel and Pixel XL owners are experiencing a complete failure of one or more of the microphones in their flagship Android phones. Google claims that less than one percent of their devices are affected; even so, a thread about the issue on the Google Support Forum has over 800 posts thus far.
A Google employee has responded with an explanation of what's going on. Here's a summary of that explanation by AP:
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.
Source: Android Police