Dolby Digital Plus is a new format for the Arc, so there isn’t much info about 7.1 specifically, but I”m pretty confident that you aren’t losing anything.

The Arc should be downmixing it to match your speaker layout.

That’s really interesting! I didn’t think that was possible. 
Do you have earc? 
What is the source of the movie ? 

As I have a TV with eARC, I can also confirm it will show Dolby TrueHD 7.1 under audio in as well. So it is showing Dolby Digital Plus 7.1 and Dolby TrueHD 7.1.

I wish when you added physical surrounds, it would turn the 5.0.2 system into a 7.0.2 system. It is sad that the “rear” speakers on the sound bar just be some part of the front left/right channel instead of becoming part of a 7 channel mix, when clearly it now understands a 7.1 audio track...

Thanks for the replies!

My TV only has ARC (no eARC). I can see the Plex dashboard showing the source “TrueHD 7.1” being transcoded to EAC3 → showing as Dolby Digital Plus 7.1 in S2’s About my System.

it’s mainly the fact that 7.1 is “not supported” that concerned me whether it was playing it correctly.

The Sonos ARC doesn’t appear to play 7.1 truHD correctly, the dolby atmos 7.1 demo is silent when it plays audio in the rear right and left speakers.

My ARC is plugged in to my TV’s arc HDMI input, and I am playing the dolby demo videos via PLEX on the most recent (2019?) Nvidia shield pro. My TV is a TCL R615, which has it’s audio set to ‘pass thru’. 

No atmos demo I’ve downloaded that is 7.1 plays anything through its “rears”.