What model TV do you have? Do you have CEC enabled in the TV and Fire TV Cube settings?

It’s an Insignia Fire TV (built in)….BUT….I also have it connected to a FireTV Cube (ethernet hard wired for speed). Yes, CEC is enabled. There is only one HDMI ARC input on the tv, several regular HDMI inputs. The Sonos Arc is plugged into the HDMI ARC slot while the Fire TV Cube is plugged into HDMI 1

Did you run the remote control setup in the Arc settings?

i.e. Settings - [Room with Arc] - Remote Control Setup

Yes...I finally got it to work! I’m not very happy with the Insignia TV hard wired for Fire TV, though….