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Only 2 clients that I know of that can do Atmos sucsessfully is Nvidia Shield TV and running PlexHTPC on windows.
I can play Atmos and HDR no issues through PlexHTPC, can't get Atmos to work with the plex app on my TV's built in Android TV with the official app, no luck through chromecast or amazon TV Stick either.
Atmos works fine for me on both Plex and VLC on my Shield TV Tube.
What is the client he is using to play? It may not support Dolby Atmos.
Even if the client supports Atmos, there are more variables.
Is it connected directly to a receiver or is it to the TV then ARC out ? some atmos can work through that but only compressed EAC3 atmos over E-ARC, not super reliable and depends on the TV and sound system.
If it's a built in app, most of them have no atmos support what so ever, though I think EAC3 atmos is supported on a couple of platforms, and only EAC3 compressed atmos, and in those cases it may be apps like netflix, prime, disney etc only, not third party apps like plex.
Ah. This might make sense. He's using an apple TV and can get atmos with other services. I also use apple TV and get atmos on my home network. But he isn't when its an indirect connection.
Plex on AppleTV is known for not supporting Atmos pass through. I have never actually tried it because I don’t have Atmos at home, but most common suggestion is to pay for Infuse on AppleTV