Android 9 issues with Pixel
Seems surely the problem of hardware i.e. microphone. I have Pixel XL, and there are no software compatibility issues that i found yet. Still if u are very much sure, then you can factory reset the phone(it clears all the phone data and fresh start the Android) by going to Settings -> System -> Reset Options -> Erase All Data. 

Note: It is recommended by many tech experts to Factory Reset the phone after installing an Android Version update as it clear unused previous OS caches.