The statements made in this answer may be true, but they are misleading w.r.t. the OP’s original question, asking about Atmos compatibility with a Sonos Arc sound bar. I just went through this, with lots of tech support and research. If you have an ARC TV set, the fault is a truly bizzare twist in ATV Atmos output restrictions
-ironically the ATV, despite what it handles internally, will output only DD Atmos True HD (ADDTHD) which requires at least a TV with an eARC HDMI, to pass TRUE HD content to the Sonos
-yes you read that correctly - the ATV which internally apparently uses ADD+, only supports ADDTHD output, it is not capable of outputting ADD+ ! (See Apple Support link at the bottom of my post)
-because the ATV will not output Atmos DD plus (ADD+), if you have an ARC (not eARC) TV set, the ATV drops the best output option down to PCM stereo.
-this is why you see on the Sonos App, that the soundbar is playing DD 5.1 when playing Atmos content from the ATV
-any TV set with ARC, using its own native streaming apps, will properly pass on internal ADD+ via its ARC port to the Sonos
-this is why when playing Atmos content from native apps in the TV set, the Sonos App will show “Dolby Atmos”
-Apple’s “solution” to this Apple created problem, is to require you to use a soundbar or HDMI receiver with at least two HDMI ports, one of which most be a full eARC HDMI input port to connect the ATV, and the other HDMI port connected to the TV set
-it gets even more complicated than Apple lets on, because while a soundbar / receiver may have passthrough ability, that does not mean it will pass through all forms of video content, e.g. Dolby Digital, HDR 10, etc. So when the ATV outputs Dolby Video content, if that is not support by the passthrough, you will get only basic non HDR video to the TV.
-Sonos soundbars have only 1 HDMI port, they do not support full HDMI in with a second HDMI port that can be connected directly to an ATV
-Apple’s own support site, states “Apple TV 4K uses a high bandwidth form of Dolby Atmos that doesn’t work over ARC connections.”
-this is the Apple support link for Atmos audio hookup to an ATV
Set up Dolby Atmos or surround sound on Apple TV with a sound bar or AV receiver - Apple Support