I’d recommend trying a reboot of the Vizio, by unplugging it from the wall for 3 minutes. 
 

There a few other things I’d recommend, if that doesn’t work. Go in to the settings on the cable box and turn off CEC on it. If they don’t expose that (no longer have access to an Xfinity box to check) then look in to one of those adapters that you can purchase to put on your HDMI cable to block CEC.

I have got two Vizios myself, and haven’t had this behavior with an Arc, but that doesn’t mean much. In the past, Vizio has pushed out a firmware update on one of my TVs that actually turned off Dolby Digital, that was painful to figure out. It’s been several years, and I hope that their QA process is better, and they’ve learned from that mistake, but it’s always worth double checking settings when you run in to an issue.

Based on your experience, I do lean more towards a CEC issue, which is why I’m suggesting the reboot first, to reset that system in the TV. 

Tried resetting the vizio again, problem still persists.

Only xfinity audio setting available is HDMI audio output, currently set to “stereo”. I’ve tried “auto detect” and “expert mode” as well. Problem still there. 
 

No CEC setting available within the box’s settings. 
 

i just find it very curious to that this problem popped up out of nowhere…

Stereo is probably a bad setting for the xfinity to be on. I’d go with “auto detect”, which should recognize your TV set, and send the appropriate signal.

Can you explain what you mean by “resetting the Vizio”, please?

Might be helpful to know which particular Vizio you’re using, so we can look at the user manual to see if there are any oddities. 

My guess is that XFinity updated the software on your cable box, of course without notifying you, and that’s what’s causing the issue to pop up now. Unfortunately, the Sonos is at the end of a very long chain of devices, and can only play what it is being handed by the TV set. 

Changed xfinity setting back to “auto detect”. Problem still ongoing. 

I should’ve clarified, by resetting the vizio, i mean that i rebooted it by unplugging it from the wall like you suggested in the original reply. 

Vizio model is D50x-G9

I am running two different Vizio models, both happen to be connected via ARC to Sonos Arc speakers, so not quite the same, but not having any issues. 

It may be time to call Sonos Support directly to discuss it.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.

I have the very same issue

VIZIO Quantum M, cuts out every couple of minute for a few seconds.

This is a disappointing system flaw, my previous Adcom Home Theatre processor never did this using the identical settings so there is no excuse for theSonos system not to decode signal off the ARC on the Visio.  I almost passed on purchasing the Sonos system because their were so many reports of cutting in and out, my thought wasSonos would engineer a solution, but apparently this is an ongoing problem that they cannot overcome.

i wish I had payed closer attention to this before buying a Beam system

As I posted before, I’d recommend two things. Make sure your TV is up to date, and contact Sonos directly. 

Sonos can only play what it’s handed. If the TV stops sending a signal temporarily, Sonos can’t play anything, no matter how much you want it to.