I'm using a Sonos Playbar to ooutput the sound for my TV from the Horizon Go box. Is it possible to program the Horizon remote to adjust the volume of the Sonos Playbar?
I assume you are talking about the Horizon Box and not Horizon Go which is the computer TV application.
To use your Horizon Box remote controller with the Sonos playbar you have to make the Sonos bar learn the remote control code using the Sonos application. This should be explained in your Sonos documentation.
Probably you could also program the Horizon remote control with a TV code that would be close to the Sonos bar but then you would probably lose the ability of switching on and off your TV with the remote.
I was able to setup the Horizon Remote Control to control the volume of my Sonos PLaybar using the instructions provided on this Sonos support page