I’d have thought updating the TV would have helped. 

The one thing I don’t see listed, which “may” help, is a simple reboot of the TV, by unplugging it from the wall for 5 minutes. When you plug it back in, it should reboot and reload the software that’s controlling the HDMI CEC connection.

However, if that fails, you could try submitting a system diagnostic within 10 minutes of experiencing this problem, and calling Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

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.

That being said, the issue is that it could be a problem in the TV set, and not the Arc, so there may be nothing Sonos can do, and it would be a call to LG’s CS group. 

You wouldn’t happen to have another TV set to test with, by any chance? Would be a great way to identify where the problem lies. 

Perhaps try this to see if it works for you.. 

• Uncable ‘other’ HDMI connected devices, just leaving your Beam cabled to the TV’s HDMI-2 Arc port and ensure that port is set to ‘Arc’ (not eARC) and audio ‘pass-through’. Direct the audio-out through the HDMI Arc port.
• If necessary, switch on both ‘Simplink’ and ‘Auto-Power-Sync’ in the TV settings.. (toggle off/on the switch, if necessary).
• Power off the TV and Beam (not standby) for a minute or two and power both back on again.
• Leave the ‘other’ HDMI devices unplugged for now (just temporarily) and see if that eliminates the issue and that the TV/Beam audio works correctly.
• If all works, then add the other devices back ‘one at a time’ to the HDMI ports and use each device for 30 minutes or so and see if any added device causes the audio to dropout. If not, you might find the issue is fixed.
• If one of the added ‘other’ devices does cause the audio to dropout, then check its device settings and perhaps switch off its HDMI-CEC control. That should ‘hopefully’ stop it from probably trying to steal ‘focus’ away from the Beam.

I hesitate to even suggest this solution, as for many months my LG CX 55 inch OLED (OLED55CXAUA) randomly stopped outputting ARC audio to my Sonos Beam Gen1 + Play:1x2] system requiring me to physically unplug the Beam’s HDMI cable from HDMI Port 2 on the TV multiple times a week. The TV would just output to its internal speakers while completely ignoring the Sonos setup until I re plugged it in. I was getting to the point of worrying about the TV’s HDMI port 2 getting worn out.

All prior suggested solutions on this website have been temporary at best, but I seem to have found a durable solution that has lasted me the past four weeks of regular usage, found by reading multiple posts about this issue and parsing together a solution through trial and error. 

Of note, I first thought this was an Apple TV / LG CX / Beam problem, but I have realized after changing multiple settings on my Apple TV 4K Gen 2 (tvOS 15.1 with output forced to Dolby 5.1) over many months that it was a red herring because I was just using the Apple TV all the time as the source.

Ultimately, I believe that that LG and Beam are not playing nice through the HDMI-CEC protocols, specifically something to do with power. Sonos and LG engineering will need to figure this out.

First, my firmware versions:

LG CX: 04.30.10

Sonos system: 13.3

Second, my fix:

Gear button on LG remote → All Settings → Connection→ Device Connection Settings → Universal Control Settings → HDMI2(Arc)  - > Manage Settings → Set Manually → Bluetooth / SoundBar → HDMI2 (ARC) → Yes to reset → DO NOT ENABLE “Auto Power Sync” (SIMPLINK is OK to be on) → Scroll down and select Sonos → Next → Done

The only thing that I have noticed different with this setting disabled is that my Apple TV remote’s power button now does not turn on the TV. I have to do this on the LG remote. For me this is a fair trade rather than having to be constantly annoyed that my supposedly first in class, rather expensive system is randomly not working. 

To confirm this fix, two days ago I went back and re enabled Auto Power Sync and I had my first random disconnection in a month today.

 At least for me, having Auto Power Sync disabled is the fix.

Hopefully this is a universal fix for others undergoing this same frustrating issue.