Hi ​@Carina Bruland, welcome to the Sonos Community!

I’m sorry to hear that you’re having issues getting your Sonos Arc working with your LG TV.  Since the TV keeps reverting to the internal speakers instead of sticking with the Arc, then it’s likely a setting on the TV itself.

I’ve had a look online and can see people mentioning a setting called “Quick Start+” which can cause the LG TV to revert to the internal speakers. The steps to disable this setting were to go to settings → general → devices → TV management → quick start+ on the LG TV. It’s possible that these steps have changed with any TV firmware updates, so please check your TV manual if you can’t find this option. You should also make sure the Simplink (LG’s HDMI-CEC function) is enabled.

If adjusting the Quick Start+ and Simplink settings still makes the TV change back to the internal speakers, then I’d suggest contacting LG support to discuss with them. 

I’m having the same issue with my new Samsung tv, does anyone know how sort this out please 

Maybe try removing all the other TV HDMI connected devices (apart from the Sonos soundbar) and see if the issue goes away. If it does, then it might be that something is stealing the CEC focus away from the soundbar. Connect back each device, one at a time, to the TV until you perhaps find the ‘culprit’ and switch off CEC control on that one device only, or use a CEC-Less adapter (if required). Hope that suggestion works for you.