Contact Logitech?
I think you’re going to run in to some issues, though. When the TV is on, the Arc (not ARC) is controlled via ARC (Audio Return Channel) data from the TV set. Since the TV is off when you’re listening to music, you’d need to convince the Logitech software to treat the Arc as a different device, and use an IP based protocol to connect to it.
I never was able to get a satisfactory response from Logitech/Harmony, and given that they’ve discontinued their Harmony line, I’m not overly surprised that there’s reticence on their part to dig in to this conundrum.
Frankly. every time I tried to add my Arcs to the Harmony, they showed up as Amps. I finally just ignored the Arc completely in the Harmony software, and told the system it was using the TV to adjust the volume, and that got automatically carried by CEC to the Arc.
But I get what you’re trying to do. I was never successful. I’d be interested if you’re able to get any type of resolution, although I tend to use a pair of PLAY:5s (gen 2) to listen to music in my living room. But just the satisfaction of having someone be successful would be enormous.
I use a Harmony hub and it had no problems finding all of my Sonos speakers when it did a device scan on my network using the Harmony iOS app. I then created an activity for just controlling the Sonos Arc when in music mode, and my Harmony remote can now control music playback with the Arc (play, pause, skip forward, skip backward, volume up, volume down).
I use the Harmony Hub/App too and a Harmony scan finds my Sonos products… the Arc in our Home is called ‘Living Room’. See attached screenshot.
But is it recognized as an Arc, or as an Amp? When I kept trying to add the Arc (rather than “finding” it), it would identify the device as an Amp. I even sent a complaint to Logitech/Harmony about it, and got a “no answer” from them.
Bruce,
The hub/software identifies it correctly in my case as a Sonos Arc - see attached.
Most odd. The best I got from them was a “we’ll pass it along” comment. Having just moved, I’m having to reset up my Harmony in the next several weeks, I’ll give it another shot.
Like I said, Harmony finds it no problem, but it doesn't really know what it is. It thinks it's a Panasonic or Skocol (whatever that is). If I don't pick one of those models, it won't add. BTW, a lot of folks getting it working are coming from a beam. I am not and I think that is part of the problem.
I’m also using my Harmony Elite remote control and it works fine with all my SONOS speakers… I remembered when I added my Symfonisk speaker and lamp to the system, I couldn’t find them after I have refreshed the system many times… and then I suddenly realized that they are actually IKEA’s products… Haha!!! 
