Loudness/speech enhancement/night mode

In that case, I suspect the Apple TV was probably only playing audio in PCM stereo before that setting change on your Apple TV. I would look closer at your TV audio-out settings, as it might perhaps be defaulting to DTS 5.1 audio rather than Dolby Digital 5.1. 

I really recommend that you check the TV user manual, to just see if that perhaps helps you to resolve the issue. You want the TV to play the Dolby Digital 5.1 codec and ideally to "pass-through" that digital sound-out audio from connected devices. If there is no "pass-through’ option to then try resolving the lip-sync issue manually with the LG TV A/V lip-sync features built into the TV settings. If the issues persist then it maybe best to speak with LG customer support.

There are other things you can do, using an audio extractor, but I don’t want to get you even more confused with going down that slightly more technical option to resolve the lip-sync issue.

Okay, sound is back.
I managed to break a piece of the optical output, so the cable was out.
Fixed it with some duct tape🤪

So the Apple TV is now on Dolby 5.1.
I don’t think the screenshots were irrelevent. I think that the “AV sync. adjust” is the one to use.
But it was a good tip that I needed to adjust it manually. I thought it was automatic.
So I put it at -5 now, which is max. And it is at least better. Not perfect, but livable. So thanks😊
The “Loudness/speech enhancement/night mode” - is still the same though.
But of course...sync was way more important.
 

TV audio settings are all greyed out as soon as I choose HDMI/Optical out.

Yes, that is usually ‘by design’ as the soundbar is now handling the audio.

Right. 
So there is no way to disable DTS.
Anyway, it shows Dolby Digital 5.1 in the Sonos app, so that’s probably fine.
It’s still out of sync. The only thing that I can adjust is “digital sound out” which is “auto” or PCM.
If I choose PCM, the dync is fine….but only stereo of course.

I would now perhaps just try fine-tuning things by ‘playing’ with your LG TV settings to get the best lip-sync possible. You could even speak with LG customer support and see what they suggest to improve the audio pass-through. See if there are any TV firmware/software updates too.

If the problem persists, then some LG users with older TV lip-sync issues and no TV ‘pass-through’ features, do sometimes opt to bypass their TV altogether by using an audio extractor/splitter, but that means buying hardware and inserting it into the setup between the Apple TV and your LG TV. Such a device extracts the audio from the Apple TV A/V signal and sends it direct to the soundbar, bypassing the TV altogether. 

The advantage of doing that, besides getting the lip-sync issue resolved, is that it will also upgrade the audio to provide Dolby Atmos audio playback from your Apple TV 4K. One such audio extractor is called the HDFury Arcana. You can perhaps read more about that product here…

https://www.hdfury.uk/product/hdfury-arcana/

Thanks, I’ve seen those.
Quite expensive. I guess I’d better go for a new TV at one point, and live with just stereo til that happens.
Too bad, I was buying the Sonos hoping it was just plug’n’play, but there’s always a weak link. In this case the TV😊

Also, now that you have switched the Apple TV to DD 5.1, perhaps go back and try the Optical connection once again, alongside the LG lip-sync feature adjustment, as you ‘might’ find you perhaps get better lip-sync using that connection instead. Some TV’s can ‘occasionally’ work better with that connection, although technically speaking, it has less bandwidth than the TV’s HDMI-ARC connection. These things though are always worth trying.

Yeah, I did. And it does work better. But it only gives me PCM/Stereo. The DD5.1 option is not there😏

It should be🤔? - that must be a TV setting - check the TV’s "DTV audio” setting and set that to DD 5.1 - it might be under advanced or additional settings in the sound-out settings area of the TV.