Set up:

• LG C1 OLED using eARC and Pass Through
• Sonos Beam (Gen 2)
• Sonos SLs (set up as surrounds)
• Apple TV 4K 2021 (sound output Auto, Atmos ON)
• Xbox Series X (using Dolby Atmos for Home Theater)

TL;DR - If you have both the Xbox Series X w/ Atmos enabled turned ON and the Apple TV 4K w/Atmos also turned ON (both on separate HDMI inputs), when you try to watch Apple TV content that comes in with Multichannel PCM 7.1 (e.g. Bob’s Burgers on Hulu), there is no sound output from the apple tv when playing the actual content. Interestingly, the navigational audio from apple tv (boop, boop, boop) works when navigating the menus (e.g. browsing Hulu), but as soon as you try to play content (e.g. start an episode of Bob’s Burgers) it will have no sound output 100% of the time.

To make it even weirder, while pausing Bob’s Burgers, if you unplug the HDMI cable from apple tv multiple times you can on occasion get audio to work altho the audio channels are then incorrect - the vocals come thru the rear surrounds only!

HOWEVER, all these issues go away, as soon as I turn OFF the Xbox Series X that’s on the other HDMI input, meaning that the sound returns to the apple tv 4k content on the other HDMI input and the audio channels are now correct with vocals in the front.

My theory is that Xbox Series X uses Dolby MAT container, so does Apple TV 4K 2021. When you have both devices turned ON and both set up with Atmos on the settings, it causes some weird clash between both devices and Apple loses the audio on content, while Xbox gets to keep the audio.

I was going crazy, about to return all my sonos equipment.

So bizarre!