Well… I think anything you did would have to be actioned by the player via the server - unless your server could somehow run commands remotely (which I don’t think is advisable).
The answer depends on whether you want a workaround or a completely integrated solution. A workaround would be a set of BAT files on the Player which had commands for adjusting the volume to certain levels. Then a set of layouts in the CMS which contained the Shell Command module, calling the appropriate BAT file.
You’d schedule the layouts when you wanted the volume to change - probably with the priority flag ticked so you knew they would run.
A full integrated solution would mean modifying the Player Source Code to control the volume, based on the settings provided in the RegisterDisplay call. Then modifying the CMS so that the user could configure that information - I am not sure how the UI for it would work, as I would assume there would need to be some events, that triggered the change. Maybe as part of Interactive Signage