Not sure if it works yet, but in
https://forum.xda-developers.com/pixel-xl/help/notification-sounds-stop-randomly-pixel-t3720585
Read jskinvest's posts from #82 down. Basically they say the faint alert tone to alert you during phone calls of other things coming in (texts, emails, other notification tones, etc.) which was introduced in 8.1 is the issue. So the work around for the time being is to (when your incoming call comes in OR you are about to initiate a call) swipe down from the top, click on Do Not Disturb, then when the call is over, do the same to return it to (likely) Alarms Only. He/She has not had an issue doing that for 2 weeks and previously had to do the reboot the phone fix every 6 to 48 hours.
https://forum.xda-developers.com/pixel-xl/help/notification-sounds-stop-randomly-pixel-t3720585
Read jskinvest's posts from #82 down. Basically they say the faint alert tone to alert you during phone calls of other things coming in (texts, emails, other notification tones, etc.) which was introduced in 8.1 is the issue. So the work around for the time being is to (when your incoming call comes in OR you are about to initiate a call) swipe down from the top, click on Do Not Disturb, then when the call is over, do the same to return it to (likely) Alarms Only. He/She has not had an issue doing that for 2 weeks and previously had to do the reboot the phone fix every 6 to 48 hours.
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