Sonos Arc not playing 5.1 only Atmos

I have the same problem with my LG OLED C3, Sonos Arc and Apple TV 4K 3rd generation. I don't have the problem with my old device, Apple TV 4K 2nd generation. It may be just me but the ATV 3rd generation seems to have a little better sound and picture. When it works I like the Apple TV 3rd generation, I just can't play any content except Dolby Atmos or stereo PCM. I've spent hours on the phone with Apple, LG and Sonos. No one seems to have a solution.

What 5.1 content is playing that goes silent? Is this from an online streaming service, or Home Sharing from media files on a networked computer?

Note that Apple TV supports Dolby Digital 5.1, Dolby Digital Plus (up to 7.1), but not Dolby TrueHD, not DTS or its subtypes. Apple TV will decode to PCM Multichannel in the ‘Auto’ format setting, not passthrough. Verify with the TV and audio system that the PCM Multichannel format is supported with current settings and configuration.

I had the exact same problem until I ensured full pass through on the TV rather than have the TV process the audio.

settings > sound > advanced settings > match screen and sound. Choose “bypass”

this is on the LG TV. No setting had to be changed on the Apple TV or through Sonos

That was one setting I had not tried, but it didn't help. I am back to using my old Apple TV 4K 2nd generation. I have no problems with 2nd generation only the 3rd generation. Thanks for the suggestion.