Can’t you just stream music to the Arc like any other Sonos component (other than Ace)?  We have a Playbase on one TV and we often play music to it.  Not sure what an HDMI dongle is or would do - what are you trying to do with it?

You could also use Alexa or SVC.

The television is in a separate room and using it as a signal source is not a good option. HDMI dongle would allow BT to access the Arc, but I will see if anyone else has an idea.

I did see an article in the Sonos community about possibilities, but can't find it now.

I’m puzzled: where’s the Arc?

The Arc can still be a speaker that plays a streamed source - radio station, music service such as Amazon Music, or a NAS or other stored Music Library. 
Or, you can Airplay to it. Or use your Roam 2 as a “gateway” - Bluetooth to it, but play out on the Arc (as long as Roam is on the network with Arc). 

Thank you. I finally muddled through the process, frankly the instructions weren't as clear as they could have been.

The TV is in a separate media room.