I am having the same issue. Had Xfinity service tech out but that was useless as they aren't trained to troubleshoot these kind of issues.
I have (4) TV's using the Roku Ultra devices and( 1) Xfinity main box in my home that were working fine for "years". A few months ago we started to have audio sync problems or audio loudness issues would occur. These audio issue are as follows:
1. TV is on for some period of time ( could be 20 minutes or 3 hours) then the audio would drop in level or increase in level to the point where it would be blasting.
2. TV is on for some period of time then the audio becomes distorted.
3. TV is on for some period of time then the video leads the audio by as much as 4-7 seconds. This is not a minor sync issue.
Temporary fixes to date:
1. When the problem occurs, we step off channel then return back to the desired channel and problem is corrected for some period.
2. Reset all devices in the chain including Asus router, Netgear modem & Xfinity box. Router and modem both certified to work with Xfinity. Reset preformed several times.
3. Had Xfinity service technician at home to diagnose problem and reset Xfinity equipment.
System setup from Xfinity demarcation:
Xfinity cable to Netgear cable modem, Cat 6 cable from cable modem to Asus router, Cat 6 cable from router to Netgear multi port switched (unmanaged), Cat 6 cable to all Roku devices.