Multichannel PCM on Arc

Not for me at least. When I make a change to 5.1 on a device like the PS5, it updates instantly in the Sonos app to LPCM 5.1 from 7.1. My Apple TV 4K works correctly, outputting 5.1 LCPM on 5.1 media without any settings adjustments (even though more and more DD 5.1 content is being upgraded to Atmos, I couldn’t even find a 5.1 movie last night) I use that surround speaker test app which is known as a baseline.  When the Sonos app says 7.1, the sound is messed up and has gaps where the 2 extra speakers should be. The bug for me is the PS4/5 thinking the setup is 7.1 without manual intervention. Are you sure this isn’t a TV pass through issue? Try that surround speaker test app in DD5.1 and see what the Sonos app shows. 

Not sure if this pertains to this conversation, but I just tested a 7.1.2 test tones video from my Dolby Atmos demo Blu-ray on my 5.1.2 Arc/Sub/Play:1s setup. The test tones for the left and right surround channels played perfectly clear out of my Play:1 rear surround speakers indicating that the Arc will take a 7.1 track and mix the two side surround channels into the two rear surround channels.

Weird, perhaps it is my LG CX, I know others reported the same that have the CX as well.

Where can I find the speaker test app please?

In the Apple TV App Store: “surround speaker check”. In the app try the 5.1 speaker check. 

For Atmos, it doesn’t seem like there is a problem with the down conversion. The issue folks are having is with 5.1/7.1 content when Sonos is receiving a Multi-Channel PCM 7.1 signal. Given it was a Blueray, the signal was almost certainly not PCM. Anything using one of the Dolby codecs seem to work great (which Bluerays use).

@FishyD I have a DTS demo Blu-ray disc coming next week that includes a 7.1 “DTS Sound Check” video. I’ll report back how the Arc handles the surround test tones from the 7.1 PCM signal.

I just performed the “DTS Sound Check” from a 2015 DTS Demo Blu-ray disc. I am playing this on a Panasonic DP-UB820 UHD player with DTS set to PCM. These are the results:
 

DTS Sound Check 5.1
All channels played the test tones correctly. The Sonos app reported “Multichannel PCM 7.1”.

DTS Sound Check 7.1
All channels played the test tones correctly EXCEPT the Left Rear and Right Rear Surround channels. The surround test tones only played out of the Left Side and Right Side Surround channels. The Left Rear and Right Rear Surrounds were completely silent. The Sonos app reported “Multichannel PCM 7.1”.

So it appears the Sonos Arc is not playing the Left Rear and Right Rear surround channels when playing a DTS-HD Master Audio 7.1 track.

@GuitarSuperstar Is there a setting on your BR player to force a 5.1 output? Just out of curiosity, that fixed this exact issue in my experience. Do ARC/eARC sound bars use EDIDs or some kind of config file to tell TVs what their capabilities are? I wonder, is the problem that Sonos isn’t giving proper format instructions to upstream devices or that the sound bar isn’t properly mixing 7.1 down to 5.1. 

I just performed the “DTS Sound Check” from a 2015 DTS Demo Blu-ray disc. I am playing this on a Panasonic DP-UB820 UHD player with DTS set to PCM. These are the results:
 

DTS Sound Check 5.1
All channels played the test tones correctly. The Sonos app reported “Multichannel PCM 7.1”.

DTS Sound Check 7.1
All channels played the test tones correctly EXCEPT the Left Rear and Right Rear Surround channels. The surround test tones only played out of the Left Side and Right Side Surround channels. The Left Rear and Right Rear Surrounds were completely silent. The Sonos app reported “Multichannel PCM 7.1”.

So it appears the Sonos Arc is not playing the Left Rear and Right Rear surround channels when playing a DTS-HD Master Audio 7.1 track.

Is the DTS 5.1 reporting as multichannel pcm 7.1 correct then, or should it be multichannel pcm 5.1?

All my devices that output 5.1 such as blurays with DTS 5.1 output as 7.1, even on my series x outputting as uncompressed 5.1 says 7.1 on the app. 

@ledzep1 He played a 5.1 test sound, it doesn’t necessarily mean the BR player was outputting a 5.1 signal. The BR player could be having the same issue as our PS4s/5s where for whatever reason it recognizes Arc as 7.1, outputs a 7.1 signal and Arc doesn’t convert to 5.1  If he can manually set the BR player to 5.1, that would eliminate that variable.   

In my experience across 3 devices now, devices outputting LPCM 5.1 show properly in the Sonos app as PCM 5.1. I think it may be your TV model/settings that is the problem there. What are your pass through settings set to?
 

There is still definitely something odd though with devices thinking Sonos is 7.1, I think. 
 

My PS5 when forced to output uncompressed 5.1, shows correctly in app as 5.1. When allowed to pick itself on “auto” it shows 7.1 in the app. 
 

Did you try that ATV 5.1 test app?