Have you previously raised the issue with Sonos Support and submitted diagnostics?

Unless an issue is raised with Sonos, they are unlikely to know of possible sound issues occurring with your existing television.

Same issue but I had no issue with the old ARC but since we have the ARC Ultra we have this issue. Also the interval is not always 20 mins, sometimes shorter.

Issue is occurring on tv apps like Netflix but also on hdmi devices or NAS.

Tried another new ARC ultra but same issue. Samung Tv is 1jr old and using stnd hdmi cable that came with Sonos and I have passthrough sound enabled.

Still have the old ARC on another tv without any issue so I still want to do some more testing but my gut feeling tells me it's a software issue

I also experience more drop-out in sound now than in my old Arc. Before it was only with uncompressed Dolby Atmos content every 20 minutes. Now I get them from TV apps like Netflix, Prime and Disney Plus as well. 
Sonos and Samsung really need to communicate and find a solution. I also think it’s a software/firmware issue. Especially with the eARC port. The dropouts are most likely some handshake issue or switching between TV and Sonos Arc Ultra at random. 
 

Do you use Samsung apps or Apple TV. Unfortunately the Arc has a long-running problem with Atmos that is encoded in the Dolby MAT format which Xbox and Apple TV use.

I use the Samsung apps. But I also watch 4K UHD blu-ray discs on my PS5 Pro. Should be TrueHD Atmos. I get the 20 minute interval between dropouts in sound with TrueHD. The dropouts on Samsung apps (Netflix, Prime, Disney+) are more irregular. I get no dropouts (so far) with PCM. But I didn’t get a Sonos Arc Ultra to NOT use Atmos. Or any other Dolby codecs.

Make sure the TV is set to these settings:

HDMI-eARC Mode: Auto
Digital Output Audio Format: Pass-Through

Then try this:

Disconnect everything from the TV and unplug the Arc Ultra and TV from power for a couple of minutes. Plug everything back in but keep everything disconnected. Make sure Anynet+ is enabled in the TV settings. Run TV Setup in the Sonos app and follow the app instructions. When the app instructs you, connect the Arc Ultra to the TV’s HDMI ARC/eARC port using the Sonos-supplied HDMI cable. After the Arc Ultra has been successfully connected, connect your other devices to the TV.

I tried this. Power cycling didn’t help, sadly. I even tried again. Still a dropout in sound after 20 minutes.

Try connecting the PS5 to a different HDMI port.

I have the same issue as above, samsung tv, sonos ultra and sound drops for a split second every 20mins or so. 

spent 40 mins on the phone with support today, tried power cycle, running diagnostics and no fix. 
tired new hdmi cables, different ports, no solutions. its almost to the point im going to return the Sonos sound bar for a different brand. I didn't spent all this money just to have it not work.