Max2Play Home 2016 (en) › Forums › Max2Play as Squeezebox (Player / Server) › Is there a global built-in volume control for max2Play multiroom setup ? › Reply To: Is there a global built-in volume control for max2Play multiroom setup ?
As far as I know, there is no master fader for all players in the Multiroom system. So you would really have to build one yourself with openHAB or via script.
In the API Examples plugin you can see the commands to change the volume of a player. You could then copy these into your script or openHAB.
Btw, in the LMS settings, you have the option of synchronising the volume of already synchronised players. Perhaps the LMS plugin „Group Players“ could also be interesting for you. If I am not mistaken, with this plugin it is possible to control the volume of groups.