I think this is/was an issue with some LG CX TV’s, see this thread for additional info:

The above link runs into a slightly longer thread, which seems to infer that some devices send LPCM 7.1 audio ‘through’ the LG CX TV if it’s HDMI sound-out is set to pass-through,  and that Sonos plan to possibly downmix the channels to 5.1 in a future update (no information about when that might happen) - at least that’s how the thread reads to me at a quick glance.

Maybe changing the audio-out settings on the A/V sending device, or changing the TV HDMI Digital Sound Out to ‘Auto’ (instead of pass-through) to let the TV process the audio before it gets to the Sonos HT, will help to fix the issue in the meantime, if it is perhaps causing you an issue.

Yes this is still an issue and an infuriating one at that. Sonos has not yet announced the downmix feature and LG are completely silent on the issue which is entirely unsurprising. 

yes its still an issue.

if you see “LPCM 7.1” in the Sonos app then 100% the sound coming from the speakers will be messed up. Right now, its needs to say LPCM 5.1 or its going to sound wrong

I thought it was in the testing as a “fix” sometime last year, funny that Sonos has now labelled it as a “feature”. 

I have got a bit lost with it all now as there was a few issues with LPCM at the time these issues were first reported, which is why it would be nice of a Sonos support rep on here to respond, by saying if a fix/feature is in the works, maybe its an LG issue, then so be it, just would like some response.

Is it really that hard?

I bought the Sonos ARC, sub and symfonisk frames for rear duties just a couple of days ago and although I am happy with the sound I am having issues regarding DTS Blu-ray. I have set my Blu-ray player to output DTS master audio as LPCM but the app shows PCM 7.1 not 5.1. Does this mean the ARC is ignoring 2 channels and not playing audio correctly? I have a Sony a80j OLED TV which is set to passthrough and have had zero issues with my previous Samsung q950t soundbar connected. I am not new to home theatre and have had various systems throughout the years from full atmos installs to various HIFI separates and sound-bars so I know it’s not user error or setting problems. I was under the impression I could get DTS Master audio via LPCM without issue but now I’m not so sure. Any ideas?

There is a known LPCM 7.1 to 5.1 down mixing issue with the Arc. Sonos is aware of this and is working on fixing the issue. Some people have reported they can clearly hear two channels missing from the surround audio, but I can’t tell the difference. I have tested the surround channels with a DTS 7.1 Sound Check on a DTS demo disc and all four surround channels play out of my two surround speakers correctly. But I am also using an HDFury Arcana which might be the reason why I don’t seem to be experiencing the down mixing problem.

I am replying to follow for any updates as this is what I’m waiting on as well. LG C1 and pass through causes 7.1 on a 5.1 system. Hoping they fix this soon!

Also very interested in some kind of update on a solution to this. 

Is this a problem for the LG G1?  Are we supposed to keep the TV audio set to auto still?  If my source file is a TrueHD 7.1 Atmos remux, do I still want my Sonos App to show Multichannel PCM 5.1?

The Sonos app should show the Dolby Atmos logo if you are playing a Dolby TrueHD/Atmos source. Be sure the HDMI Input Format setting on the TV is set to Bitstream and eARC is enabled. If it is a Blu-ray disc, set the player’s settings to Bitstream too and turn off Secondary Audio.

This is on a 2021 Apple TV 4k with Plex.  I’ve tried the Plex app and with Infuse 7.  The TV is set to Bitstream and eARC is enabled.  I’ve tried with the TV set to both passthrough and auto.  Auto show’s multipath PCM 5.1 on the Sonos app and passthrough shows multipath 7.1.  It’s the same with any other source file I try irregardless of the sound format (unless it’s a stereo 2.0 source).

The Apple TV 4K doesn’t support Dolby TrueHD audio. You need an Nvidia Shield.

So since the Apple TV converts the TrueHD audio, do I want it to be Multichannel 7.1 or 5.1 on the Sonos app?  Is the 7.1 problematic on the LG G1 or was that problem just with the LG CX TV’s?

From what I’ve read, I think the G1 has the same issue as the CX.

When you listen to the Multichannel PCM audio in 5.1 or 7.1, do you hear a difference? Is there any audio missing from the surround channels?

I don’t think there’s any audio missing, In the rear One SL’s, I still hear music and sound effects from the movie files I’ve played.  Correct me if I’m wrong but from what I’ve read, the full Sonos setup (Arc, Sub, 2xOne SL) is hardware limited to 5.1 anyway?

Yes, the Arc, Sub, One SLs setup is a 5.1.2 setup.

If no channels seem to be missing from Multichannel PCM 5.1 or 7.1 audio playback and it sounds good to your ears, then that’s all that really matters.

After tinkering with settings tonight, I found that if I turn off Atmos on the Apple TV and set it to DD5.1, and set the LG G1 to passthrough, I get the Dolby Digital 5.1 text on the Sonos app instead of Multichannel PCM 5.1 and the sound was much better with way. But if I play an Atmos title, I’ll have to change the settings again.

Any chance of an update from Sonos on this? Even if to just confirm that it isn’t a problem? 

It’s still a problem especially with an Apple TV and Sony PS5. I just set mine to output in Dolby Digital 5.1 now, it sounds really bad on PCM 7.1 in comparison. The only device that works well is the Xbox series x as it outputs all game audio in atmos so has no problems. 
 

I have the Sonos Arc full setup with LG C1 and Apple TV. Weird and bad stuff going on with sound as well. Someone mentioned a known bug with Apple TV with 15.3 but 15.4 is supposed to solve (no Atmos only MC 7.1). Have mine as pass through, bitstream and atmos on. 

depends how you are testing it too. When I found this problem I was testing it using a test disc that plays isolated beeping sounds through each channel one by one (it's a calibration tool so you can make sure the channels work and the volume is correct) and two of the channels were clearly not working, the disc was playing audio but the Sonos speakers made no sound. I also tested it a second time using a game - games also works well for testing because you can make a static sound come out of any channel you want by moving around.

Testing this problem using a movie or music or anything that you can't control won't work  because you have no idea what you're supposed to hear and just accept it as fact. Testing surround audio in a movie or music is only possible if you have a 2nd system to compare against 

Are they?

Sorry to quote an old post, but this issue is ancient now, its quite laughable that they said they were working on it, which is well over a year ago. They dont even acknowledge the issue or respond to posts on the subject anymore to give an update. 

Issue or no issue, they should respond.

Hi @ledzep1 

Sonos does not currently support 7.1 channel McLPCM audio, though, as you see above, some users report experiencing no problems with it.

The official line is, “this is a feature we are currently working on, however; we do not have any further information to share with you at this time. Rest assured, we will address this in a forthcoming update.”

Once we have made our position clear - that we have never supported this format, therefore it not working isn’t an issue or a bug, but a forth-coming feature that we haven’t yet released or advertised - it seems needless to repeatedly answer all references to it. When we do release the feature, we’ll update the relevant threads on the forum - closed or not. Incidentally, I don’t think “working on” literally means there’s someone coding away at it right now, but that it is in a list of work that needs to be done and is quite likely to be repeatedly pushed down that list by actual bugs/issues that show up, or by upcoming projects/features that require a lot of concerted effort.

If Sonos doesn’t support 7.1 LPCM why/how does it report it in the app? I don’t think anybody is deliberately trying to send an unsupported format to the speaker, or waiting for it to be added as a feature. Rather it appears that the speaker is either asking for it incorrectly, or it is being sent incorrectly over eARC by LG OLED TVs. Either way this is a problem not a missing feature albeit maybe not of Sonos making. The frustration in these threads is more to do with the lack of clarity and uncertainty of whether it is impacting the sound in any way. I’m inclined to suspect that a hardware issue has been found hence the silence for so long and the repositioning of this as a feature but would be happy to be corrected.