On all home theatre devices, not just Arc. 

I’m not and engineer nor software designer…

I agree that what you are asking for would be a very useful/welcomed addition to all post Playbar/Playbase/Beam (gen1) soundbars. However, as a layman it would seem to me that more is required than can be accomplished by a firmware update as the internal controller board would have to have the requisite nodes to accommodate individual volume control.

Again from a layman’s point of view IMO it’s akin to adding individual volume control to Sonos Architectural speakers powered by a Sonos amp. It can only be accomplished (at present) via 3rd party integration devices. Maybe someone more learned than I can offer an opinion.

On a high note I can image Sonos offering such control on future iterations of the Arc, Beam 2 and Ray. In the form of an Arc2, Beam3 and Ray2 respectively. Then again Sonos may have a completely different nomenclature for identity purposes; such as was the case with the Era Series that replaced the Sonos One’s.

This request has been asked too many times to count and pass experience dictates that Sonos listens and accommodates when possible. So fingers X’d for the future 

Not knowing the internals I’d still hope the volume control could be done in the digital domain with a signal processor / DSP chip tweak.

Completely agree… Also such already is possible for the rear channels, sub and high channels. So obviously it „just“ seems to be a question of dsp individual level mapping for the different channels / speakers.