Apple TV hd4k - sonos arc + surrounds - LG 70up70 cant get surround sound

Apple TV uses the Dolby MAT container and that requires eARC as it needs the higher bandwidth. If your TV only has the standard ARC port then you will either have to set it to change format so it can force it to use DD 5.1 or use an adapter like the HD Fury Arcana, which bypasses the TV in order to connect to eARC.

Thanks for posting in the Apple Support Communities Nordlead_Klo,

According to the following resource: Play audio in Dolby Atmos or surround sound on your Apple TV

Apple TV 4K and Apple TV HD automatically play sound in the highest quality that's supported by your home entertainment system setup and that's available with content that you watch.

If your home entertainment system doesn't support the audio format that Apple TV automatically selects, you can manually change the audio format. Go to Settings > Video and Audio > Audio Format and turn on Change Format. From here you can choose the following:

Dolby Digital 5.1: Plays audio on multiple speakers and a subwoofer, like a 5.1 speaker setup.

Stereo: Supported audio quality for all televisions, movies, and TV shows that plays sound through a right and left channel.

If the speaker is AirPlay compatible, have you tried setting it up as an AirPlay speaker from your Apple TV to see whether you're able to play back audio in surround?

Cheers!