The Sonos Arc contains the front left/center/right and height TV channels and the two One SL’s are used for the rear left/right channels - and the Sub is obviously for low frequency audio output.
When setup as a Home Theatre, all devices mentioned are bonded together as one single entity, or Sonos ‘Room’, which collectively gets tuned to the entire physical room (using the mic from a compatible iOS device). Rear speakers can be switched ‘off’, so can the Sub, but you cannot switch ‘off’ the Soundbar (master device) to leave the rear speakers etc; playing stereo audio from behind. I’m not sure anyone would want the soundstage coming from behind them anyway?
Moving on then from TV immersive surround sound audio. For music, you can play Dolby Atmos Music audio (Available from Amazon Music HD/UltraHD etc.) or you can play the usual stereo music audio (HD/UltraHD) up-to 24 Bit (48 kHz) in either ‘Ambient’ or ‘Full’ mode. See the link Here
It is possible to unbond the two rear speakers using the Sonos App, but as you mention you then lose the Trueplay settings and will need to retune each component - so personally speaking, I would give that a miss and leave everything bonded, as I mentioned there’s little point to having the soundstage behind you anyway.
In my own personal case, I chose to simply add another pair of Sonos speakers (Sonos Fives set vertical) to the same physical room, placed out in front and will often use those for music instead, but in all honesty the Arc has a wide-soundstage anyway and I think you might be happy with it and the surrounds, playing music in ‘Full’ mode anyway. Atmos music audio is also very good too, that’s if you decide to subscribe to Amazon Music HD/UltraHD.
Anyhow, I hope the little bit of summary info. here, assists you with your purchase decision.
I am so glad you answered Keith - I’ve read a lot of your other answers on this community area and you are clearly the best in the business!! Thank you.
OK - so you’ve answered my question and I also wasn’t aware of Atmos /HD music so I’m going to go for the Cinema system and see how it goes.
I really appreciate you taking the time to answer - top man.
thanks
Andy
Ah that’s ok Andy,,. always happy to to try to help out a future fellow Sonos user. I’m sure you’ll enjoy the mentioned Sonos HT setup.
If you do have any questions a little further down the line, then just post back here, as either I, or other community users are usually lurking around and about here, often trying to help.