A few things to try (but waiting on a LG update may be the only option)
HDMI cables: people will buy great TV and high rated soundbar but then connect them with the cheapest HDMI cables they can get. If this might be a possibility with any of your HDMI cables, get some great ones. I suggest overkill with 8K HDMI (
example) which might cost $5-$15 more than ones rated for 4K.
ARC doesn't have HDMI out, so you don't have an option of AppleTV to Soundbar To TV... UNLESS, you inject a Receiver with dual HDMI OUT jacks into the equation. Then it's AppleTV HDMI into Receiver, Receiver HDMI video to TV, Receiver HDMI audio to Arc. Since this would basically bypass any CEC flakiness in the LG, this would almost certainly solve all issues, while bringing the bonus of being able to hook anything else you ever want to your system to use the same audio options.
Another way to fix the volume control issue with the AppleTV remote is don't lean on CEC for volume. Instead, you can train the volume button to use IR, which works with Sonos. You do this in "learn remote" menu in the AppleTV menu. One of the options is basically to learn volume to the AppleTV remote. The "catch" with this is you must first get another remote programmed for Sonos IR controls, so that you can then use it to "play" the IR commands so the learning AppleTV remote can "see" them and then replicate them. Alternatively, I believe you can also "learn" ANY volume up/down signals with Apple remote and then have Sonos (via Sonos app) to "learn" volume up & down from whatever IR codes AppleTV picked up.
There are a variety of references online about LG TVs seeming to just drop CEC settings from time to time, so it is likely a bug that they will hopefully address in a software update at some point.
Another "one remote" option is stealth two remotes: use an iDevice as your remote control. With AppleTV remote app and Sonos app on that iDevice, you can quickly flip between apps to control them
separately (but within the same devices). There's probably an app for the LG TV too if you want all 3. If you have an old iPhone or iPad, perhaps give it a new job as your master remote (until perhaps LG addresses the bug).
And, lastly, a Universal learning remote with macros can be a great alternative "one remote to rule them all." That's what I use and it basically takes the CEC functionality goals away from the tech and puts it in macros that you can run. By this method you can basically turn flaky CEC OFF in the various devices and then simulate it with one of these kinds of remotes. The (learn) programmed ON button turns each piece on, volume manages speakers, other programmed buttons manage other features that I sometimes want to use/access, (learn) programmed OFF button turns it all off. My preferred remotes are from Universal Remote Control Inc. but there are several of these "learn remotes with macros" available. It's a bit of a task to teach them how to do the CEC stuff but then you are in control of those steps and can know for sure what commands are trying to execute when you push buttons on the remote.
