See my response to this post:
 

So just to update this I played some 5.1 and 7.1 audio test files through the beam (airplayed to the Apple TV) and in both cases the correct channels were played (the additional surrounds for 7.1 just seem to be played exactly the same as the normal R/L surrounds) so it doesn’t seem like there is any issue with the TV pushing a 7.1 channel signal to the Beam. I had read some other threads about the LPCM signal not being down mixed correctly but that didn't seem to be the case for me.

Overall I suspect that the issue is with the CX but I’m guessing it’s a quirk of how the TV processes the signal rather than a bug but there is a firmware update due imminently for the CX so I will check again once that is available.

This general topic does seem to have been discussed online a lot, even down to a tweet from the Sonos CEO but as far as I can tell any update on the issue from Sonos seems to have stopped a few months ago so it would be good to know if this is because the issue is considered to be solved, or if there is more to come. Thanks

Just bought an Apple TV and came across this issue with the sound on Sonos being read as multichannel 7.1

It’s unfortunate that this affects not only my PS5 but my Apple TV as well. Sonos needs to speak up on this fix. It has been almost a year.

I’m surprised to hear your test file results. I’ve found that audio is considerably muted when it’s being pushed as 7.1 as opposed to 5.1. I have the C1 so not sure if that’s the issue. I have to set audio passthrough as “auto” on my TV. I really hope either LG fixes their eARC issues or Sonos starts downmixing 7.1 to 5.1. It’s super frustrating. 

So a late update to this but I’ve seen a lot of threads popping up recently about this issue and I’m amazed that there’s been no reply from Sonos on such a widely reported and discussed issue?

I’ve done a bit of further testing using test tones from https://www2.iis.fraunhofer.de/AAC/multichannel.html airplayed to my Apple TV 4K. Interestingly I’ve found that the .wav version of the 5.1 test file does not play back the rear channels at all, but the 7.1 .wav version does (The AAC versions of both play back the rear channels properly. I then went and found a few more examples of .wav 5.1 files where the rear channels don’t play either. I’m not sure if this means anything? The reason for coming back to this is I’ve noticed that I do generally need to turn the volume up quite a bit higher when receiving LPCM from the Apple TV similar to the previous poster.

So another bump and update, I’ve retested the 5.1 wav test files and they now play back correctly so maybe something to do with channels and lpcm has been fixed? Hard to tell what though as the CX, ATV have also had software updates. Would be great to get some kind of update from Sonos though as I’m holding off buying surrounds until I’m confident there is no issue with this.

Just to update on this it appears to not be an issue with the 3rd gen ATV 4K that was just released. I get multichannel PCM 5.1, not sure if this was fixed by software update for older gens or a change to the HDMI output on the new box as I hadn’t checked for a while? Anybody else seeing this?