Just an update. 
after reading the other post from Chris and the reply, I turned the tv off at the socket. Unplug all the HDMI’s

HDMI 1-PS4 

HDMI 3-PS3

I left the HDMI 4 in which was my Virgin media V6 box 

I then plugged back in the HDMI 2 eARC & hey presto it worked. However, I’ve plugged back in the PS3 & PS4 and it’s gone back to the TV internal speakers again ‍♂️
What the Hell is going on here??

!Further update!

I have figured out where the problem lies, it’s when I connect the PS3 to any HDMI on the tv. 
Ive tried numerous HMDI cables, even the latest  2.1 120hz hdmi

I’ve tried everything, even changing the HDMI setting to from Bitstream to PCM, as soon as I connect the PS3 to the HDMI (when it’s switched off) the sound goes off and reverts back to the tv’s internal speakers.

I’m now at a loss as to what to do, apart from remove the HMDI from the socket every time I went the sound bar on, which is an absolute joke if I’m honest. Is it because it’s a last Gen console, I’ve changed the settings on the PlayStation 3 also, to no effect. It’s the connection to the Beam sound bar even when the PS3 is off that’s baffling

Does anyone know a workaround this problem please?

You could try using a CEC-blocker on that PS3 input, something like this;

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

The TV will then not notice the PS3 is there.

Hi

great, thanks for the reply, just ordered one on Amazon U.K., seemed cheaper that the US surprisingly! 
It arrives Saturday 19th December (2020). I’ll keep everyone posted to see if it will help anyone with the same issue..

Update:-the Lindy CEC blocker arrived and unfortunately did nothing, it did Not work. 
 

• In the LG Device Connector under Bluetooth/Soundbar - for HDMI-2 - ensure your Beam manufacturer is set to Sonos (you may have to search for the manufacturer name to correctly set it up). 
• Ensure ‘Simplink’, ‘auto-power sync’ and ‘auto device detection’ are all switched on in the TV Connection/HDMI settings (in fact I would just toggle ‘simplink’ off and back on again).
• In the TV Settings/Sound/Sound-Out make sure eARC is switched off and the Digital Sound Out is set to ‘pass through’ on the HDMI-ARC port.

And see if that resolves things for you.

Hi Ken

Thanks for your reply, unfortunately I have done everything you’ve mentioned, including using the Lindy connector and without using it. It’s exactly the same.

I’ll be honest when unplugging the PS3 to try the Lindy adaptor in the PS3 HDMI, the Sonos Beam worked a treat, within about 10-15 seconds upon plugging the PS3 HDMI back in the sounds goes back to the tv internal speakers. I’ve noticed another product similar to the Lindy, however it’s a 25cm cable adaptor but I’m not going to bother as I doubt it’ll work either. 
I cannot believe there’s no fix for this, a £1300 tv, £400 sound bar.. PS4, Virgin UHD box works great, PS3 nope .. the only option I have at the moment is to keep unplugging the PS3 HDMI every time I want to hear the sound bar which is quite frankly ridiculous ‍♂️ 
Bedsides I don’t want to knacker the HDMI slot in the tv either by continuously doing it

Hi Ken

sorry, just to add to your answer further, the LGCX55 OLED doesn’t even have Sonos in its list of manufacturers, I have to type ‘Sonos’ in on the add a manufacturer label & then tick on my typed in word. Love the tv but these problems are disappointing to say I’ve paid out hard earnt money for a broken product, I’ve also done the update on the day of arrival which was supposed to fix any of these issues amongst others  

Note you can actually switch off HDMI-CEC on the PS3 - just goto "System Settings/Control for HDMI” and switch that off. See if that helps.

Yes, I did mention you may have to search for the ‘Sonos’ manufacturers name - Its like that for many other manufacturers too as the LG ‘connector’ display doesn’t cater for every single manufacturer on the main screen. Hopefully switching off the HDMI control on the PS3 will help. I would also keep a lookout for TV firmware updates too.

PS3 Control turned off

CEC turned off 

still doesn’t work

So what is the setup - An LG CX55” with the Sonos Arc on HDMI-2 with Simplink and auto-power-sync switched ‘on’. The HDMI port set to eARC and pass-through set to ‘ON’ rather than Auto?

Then you have the PS3 / PS4 (with either CEC disabled/or using the Lindy adapter) on the other HDMI connections.

...is that correct? Or do you have anything else attached to the TV?