Have you set music playback to full under the surround settings?

Yes, forgot to add that in my original question. Music playback is set to “full” under surround settings. I have added the clarification in my original question. Thanks!

I’ve been doing further testing. I removed everything and left only the Arc. When I play music with just the Arc, I can hear a rich full sound coming from just the Arc (obviously). Then I reset the Sub (unplug, plug while pressing the Connect button) and pair it with the Arc. The moment it’s paired to the Arc, the sound is thinner (no bass coming from the Arc anymore) but I can’t hear anything from the Sub.

So it’s as if the Arc is trying or thinks it’s sending audio to the Sub, but no audio is coming out of it.

I know the Sub works, so there must be something I’m missing?

Try enabling wifi again to make sure that's not an issue?

Thanks. Will try and report back.

Enabling wifi does not make a difference. I’ll try hooking up a TV to the Arc and see if it makes a difference. I don’t see why this would be the case though.

I should have mentioned disconnecting the ethernet cables. Some modem routers have an issue. Something like RST settings? You get a packet storm effect going sometimes.

Can't see a lack of a tv being an issue.

So this is super strange.

Before moving the upstairs TV I decided to re-add the Arc to my system (reset, then re-add) but moved the Sub to the FRONT of the room. Before, the Sub was positioned at the back of the room, where the back surround speakers are. I disconnected the Sub, moved it to the front so it sits close to the Arc. I plugged it in, wired via ethernet.

Lo and behold, it works flawlessly with the Arc. 
So I know it’s not wifi. Also I know the ethernet jack in the back of the room works because the Sub works when used with the Ones but not the Arc.

I unplugged the Sub again, moved it to the back, tested with the Arc, it doesn’t work. Repeated again moving it to the front, it works.

What gives? Does the Arc knows somehow that the Sub is in the back and it doesn’t like it? I mean it’s a looker I know, but that’s just vanity at this point…

I’m going to try hooking up the surrounds now.

I’m finally done. Thanks Silverthorn for sticking with me! I’ll describe what I did in hopes that it might help someone else.

Silverthorn was right, the TV is not an issue. I don’t have a TV hooked up yet, and the Arc, Sub and Ones are working together.

Before describing the issue, it’s worth noting that I tried with wifi enabled, but didn’t disconnect the ethernet cables. It might have worked but that wouldn’t be a solution in my case because I can’t use wifi reliably in this room. Also I simply prefer ethernet.

First I moved the Sub to the front of the room. It immediately started working. I tried hooking up the Ones in the back of the room, and though they could be added as surrounds, no sound would come out of them. Tried restarting to no avail.

It got me thinking that it might be something about the network switch where the back of the room is hooked vs the switch at the front (they are different switches). The switch at the front is a gigabit switch. The one in the back is an old router converted to a switch.

So I got a long cable to hook ONE of my surround speakers to the switch at the front, to test my theory. When I was in the process of connecting the new cable, I accidentally pressed the “play” button on the speaker. Music started playing in all speakers except the single One SL still hooked to the old router.

So I replaced the router for another gigabit switch, and it worked!

Mind, everything worked in isolation while it was hooked to the old router; the Ones+Sub worked flawlessly connected to the old router. The problem was just between speakers in one switch and speakers in the router (acting as hub)

Yeah, networking issues are often where the issues are. Glad you sorted it out.