What TV, external media device, and app are you using to get Dolby Multichannel PCM?

The first number of the number of audio channels.  Second number is the number of subs.  If a third number is there, it’s the number of atmos speakers

2.0 = stereo

2.1= stereo w/sub

3.0 = left/right/center (LCR) front speakers.

5.0 = LCR + left and right rear surrounds

7.0= LCR+ 2 rear surrounds + 2 side surrounds

So an Arc, w/2 rear surrounds and 2 subs = 5.2.2

Was that the question?

LG CX, Apple TV, Full Ar setup with Sub and Ones

Sorry for my lack of detail. The show I was watching on HBOMax was labeled as 5.1. On Sonos it’s labeled as Dolby 5.1 Multichannel PCM. I guess my specific question in this case is why is all of that and not just 5.1. 

@Jeffrey_35 Streaming apps like HBO Max and Netflix typically stream audio in Dolby Digital Plus 5.1. But since the Apple TV 4K uses a Dolby MAT encoder, it plays the Dolby Digital Plus audio as PCM audio. So, ideally, the audio format you want to see displayed in the Sonos app when streaming from the Apple TV is “Multichannel PCM 5.1” from Dolby Digital Plus content or “Dolby Atmos” from Dolby Atmos content. Stereo content from the Apple TV should display “Stereo PCM” in the Sonos app.

The reason why you are seeing “Dolby Multichannel PCM 5.1” is because the Digital Sound Output setting on your TV is probably set to Auto rather than Pass Through. Because it’s set to Auto, the TV is converting the Multichannel PCM audio from the Apple TV to Dolby Multichannel PCM. If you set this setting to Pass Through, the Sonos app should display the correct “Multichannel PCM 5.1” audio.

Is there a difference between “Multichannel PCM 5.1” and “Dolby Multichannel PCM 5.1”? I doubt it. Pick the TV setting that results in the audio format that sounds the best to your ears.

To tell you what format 5.1 signal you are playing.

It gets worse, Snip from a Denon 13 channel AVR page: (you pick 13 of the 15)

outputs for 15 speakers (Front L/R, Center, Surround L/R, Surround Back L/R, Height1 L/R, Height2 L/R, Height3 L/R, Height4 L/R)

Found this link: https://hometheateracademy.com/bitstream-vs-pcm/

Basically, if I understand this correctly.  It means that your TV or source device is taking dolby 5.1 and converting it to the separate audio channels as PCM before it reaches your soundbar (or whatever audio you’re using).  So the soundbar doesn’t do any decoding.  If it was just dolby 5.1, then the dolby codec is sent to your soundbar, which will then do the decoding.

But, I could be off on this.

4 sets of atmos speakers is pretty crazy.  I’m not sure what sort of room, or content, you would actually get a benefit from that.