Unfortunately, in this manner, the Sonos is a ‘dumb’ device. If it receives a signal on the line in, it plays it. If there is no signal playing, it goes in to ‘low power’ mode after a short period of time. I would certainly hope LG could ‘fix’ this issue in an update relatively soon, since this issue would be across all devices, not just Sonos. 

I have a LG OLED B8 with same issues - has anyone managed to find a working solution?

I’m not certain, but this is usually caused by audio pass-through on the HDMI ports, which I know on the later LG C9 TV can be switched off. I think LG may have fixed the matter on the C9 recently with the v04.80.03 firmware update, so perhaps see if there is a similar update for the LG B9. It’s definitely an LG matter rather than a Sonos issue.

On the C9 TV the pass-through setting is located within the TV ‘Settings/Sound/Sound Out/HDMI ARC/Digital Sound Out’ so perhaps check the B9 TV to see if the setting is in the same place and if relevant switch it to ‘Auto’, or perhaps some other available setting’ and see if that resolves the audio issue.

Thanks Ken - I switched the digital sound out setting from PCM to Auto and checked the firmware (its the latest v05.10.05). Beam turns off with TV and then comes back on for a couple seconds before turning off. Sounds like i’ll probably just have to live with it - haven’t noticed it staying on permanently but duration seems to differ. TV Model is LG OLED65B8SL.

If you have other peripherals connected to the TV, I would see if the problem persists when you disconnect their HDMI leads from the TV … if it stops the issue, then find which connected box is causing the problem and see if disabling its HDMI-CEC control fixes the issue. 
 

Note: Some devices do not have options to switch off HDMI-CEC in their settings in which case you may need to look to a CEC-less adapter, as shown in this link (just an example):

https://www.amazon.com/Lindy-HDMI-Adapter-Female-41232/dp/B00DL48KVI

If none of the things work however, I would contact the LG support desk and explain/describe the issue to Staff there and see if they are aware and maybe plan to fix it in a firmware update, or see what they can suggest.
 

Hope you manage to get the matter resolved. 

One last question, if I changed to optical and this solved the issue - is that a sensible route to take or would I be losing out on other functionality or quality of sound?

You should not lose audio quality, but you will lose the HDMI-CEC controls. So Alexa/Google Assistant, for example, will not be able switch on/off your TV, or adjust volume etc; using voice instructions via the Sonos ‘Skill‘. Otherwise I think that’s one solution you could choose. 

I've recently connected my Beam to my LG TV and I have the same issue. Sound from the Beam stops when the TV is powered off then resumes after 2 seconds and continues for an arbitrary period of time.

I guess it's not harmful - just annoying - but if anyone has a solution I'd be keen to hear!

My TV is an LG OLED65B8SLC.