Atmos audio not playing through eARC

Quick update:

Was about to try a full factory reset on the ATV which would have been a pain and would require a re-install of a few apps with the whole password rigmorole etc.  Decided to try one last thing.

Reset Video Settings

This worked!!  Now able to select and play Atmos programmes AND sound bar shows Atmos being played. Hopefully this fix will stick!  What I can’t figure though is what caused the system to break and why re-setting previously working video settings somehow re-enabled Atmos sound?

you are not alone. Apple TV 4k 2021 128GB WiFi+Ethernet (using Ethernet, static IP with firewall rules to allow for this IP over 450/12Mbps Internet), 65” LG C3 OLED, and Vizio SB36512-F6 Atmos soundbar on eARC. Exact same issues, be it tvOS 17.2, 17.3. Run any app and a Dolby Atmos movie or show, no voice track at best, dropouts and stutters at worst. Disable Atmos and Dolby Digital 5.1 is just fine. Replaced all my HDMI cables as well for the soundbar and Apple TV.

I bought the Apple TV specifically because it supports Atmos (on paper). However it is clear that at this point, it does not do so properly. I like everything about the device other than this, and it’s currently a major drawback I was unaware of.

I noticed this problem today, after the TVOS 17.4 update, up until which (I think) everything was working fine.

I have two Apple TVs (both 4K), each is connected directly to Atmos capable sound bars which are connected to the TV eARC HDMI ports. I first noticed the problem after the 17.4 update when I started watching a programme on the Apple TV+ app but had no sound. I checked and the same happened with other apps, Netflix, Prime etc.

On checking the audio settings, both ATVs remained set to "Change Format" off and "Dolby Atmos" on. Everything had been working fine on these settings before. Now, the only way I can get sound is to select "Change Format" and select Dolby Digital 5.1.

It seems that the Apple TVs still recognise that the sound bars are Atmos enabled as they are allowing the setting of Dolby Atmos to on. When set like this, on Netflix I can see the Atmos label against relevant programmes but get no sound when playing them, there's no sound coming from any non Atmos programmes / movies either.

I've tried doing restarts, full resets and changing HDMI cables etc etc etc but, given the fact that everything was fine up to two days ago, I've come to the conclusion it's an update induced bug. I read somewhere that Apple changed their Atmos protocol to 8.1 in a recent update whereas many / most devices are 7.1? How right that is I don't know and I get bored quickly of trying to hunt for the causes of problems that, up until an update was installed, didn't exist! Very frustrating.

I've never been THAT impressed with Atmos anyway TBH, point is though, things that Apple claim the ATV supports should work!

I found the solution to my problems.

A lot of soundbars that are 2-5 years old support HDMI ARC —but they do -not- support eARC (enhanced ARC) which is higher bandwidth. AppleTV is high bandwidth and so is Atmos. Without that higher bandwidth, one is limited to Dolby Digital 5.1.

With an ARC setup, this means you’ll have to connect your sound bar to ARC, and its other HDMI port either to another HDMI port on your TV, or to the Apple TV as pass through. The second is less desirable, because your sound bar may not have the bandwidth to pass through a full 4k picture if it is a few years old.

I resolved my issue by getting a soundbar that supported eARC rather than ARC. My Apple TV now works perfectly with Atmos, no missing sound channels.