Disconnect the Arc Ultra from the TV and redo the TV Setup process in the Sonos app.

Is this something I’ll have to do often? Seems somewhat annoying 

You shouldn’t have to. But Samsung TVs are known to have issues.

Do you have any other devices connected to the TV?

I am also having this issue. I have had the arc ultra for under a week and have had to reset it up 3 or 4 times now as the speaker just no longer exists in the app. As soon as I have the app look for new products it finds it but it’s annoying to go through the setup process each time. It’s has great sound when it works. 

Are you also using a Samsung TV? If so, if you disable HDMI-eARC Mode on the TV, does it make any difference?

I have a very similar problem. At first, I tried to disconnect every device from my home theatre setup, but that did not help. Restarting Hi-Sense projector, Apple TV, Sub, Era 300 did not work either.

Since then, I realized that I have to power off the Sonos Arc ultra and power it back on. This fixed the problem every time. Strangely, that issue did not happen with my Sonos Arc SL, both on my TV or my Hi-Sense projector. 

I believe this is a bug with the Sonos Arc Ultra. Without powering the soundbar off and on, I seldomly lose Dolby Atmos, and then everything works in PCM2.0 even though Apple TV and projectors are properly configured (passthrough, eARC).

Basically now it hasn’t completely lost its settings in a week.  The main bug I am experiencing now is that if the apple TV goes to sleep or plays the screensaver, it forgets volume and everything is silent.  I can back in and out of apps on the apple tv for a minute or two and it picks back up the sound but it is always reset at zero volume.  I am going to see if setting the apple tv to never sleep helps keep it from forgetting. 

​@Delay I also experience this as well. It is pretty annoying, and again just on the Arc ultra. I start a movie, back out, go into Netflix again, and then the volume is back.

In a nutshell:

1. If I lose Atmos => power off and on the Arc Ultra
2. If I lose volume => leave the app on Apple TV, going in the app again, resume playing

Both happens when everything goes on sleep.

There could be multiple issues. 

First, disconnect all HDMI cables from the TV and power down the TV and soundbar for at least two minutes. Next, allow the TV and soundbar to reboot, but wait until the reboots complete before connecting the soundbar HDMI. Finally, reconnect other HDMI cables, if any.

Now we may need to deal with the nasty part. In addition to audio and video on the HDMI connection, there is CEC (Consumer Electronic Control). While the CEC goals are admirable, the details are poorly standardized. The goal is that the operator simply drops a disc into a player and all of the appropriate devices power up and select inputs. My favorite little glitch is the early morning cable box update. As a user convenience the final step of the update is to turn the cable box power ON. Unfortunately, CEC interprets this as a user request to watch TV and the user is awakened.

I suggest that you insert a CEC blocker in the AppleTV HDMI connection. This will prevent the AppleTV from attempting to dominate the CEC. Because the AppleTV CEC is blocked you’ll manually need to select it, but this is a small price to pay for system stability. You’ll also need to fuss with how the AppleTV remote sends Volume commands to the soundbar.

Thanks I will give the CEC blocker a try and report back.  I did try disabling the screensaver on the apple tv, I don’t know if that would prevent it from sleeping though and that didn’t work... still no sound on resuming.

​@buzz Thanks for the insights on CEC. I will consider this if it does not improve. I read on the subject and it can indeed improve stability to the price of convenience. On my end, I was happy to ditch all remotes and only use the Apple TV remote.

What leads me to think it is a bug with the Arc Ultra is that I have had the setup with an Arc SL for over two years, and I have two different rooms with the same Apple TV 4k model, and I’ve never had any issues. Now switching to the Arc Ultra, which is the only variable in my equation, and I face those recent issues.

Thanks so much @buzz !  The CEC blocker seems to have solved the issue!  I just got it today but after installing that CEC blocker the sound now works.  It no longer is being handled by the apple TV it seems as that has a different volume UI which is fine because it now works with no issues! Also plays atmos content fine so it doesn’t degrade the audio signal.

I placed the CEC blocker on the actual apple TV hdmi cable as instructed in the directions.  So if you need to control multiple devices you will need one per device.  I do everything through the apple TV so one is enough for me.

Anyway it is great that a $15 device seems to have saved the day! Thanks again for the suggestion!

The CEC blocker only somewhat fixed my problem. Basically if the Apple TV or my projector goes to sleep and volume is reset to zero. The good news now is it does recognize the volume buttons so I can turn it back up so I can hear it again but annoying. I wish Sonos would just keep the level that it was when the device sleeps. Honestly I only occasionally watch TV in the evenings so doing this once a day isn’t that bad but if I used the system more it would be really annoying. On the plus side I never have trouble hearing what people are saying like with my previous system and I have never even needed to use the voice enhance feature and it sounds great. Hopefully they will get the bugs worked out eventually.