I used VNC on iOS to remote into the desktop as control, but it is still a clunky solution. Remote desktop is similar. You still have to use mouse action (emulated on the touch screen) to control it, in addition to making sure that the volume control function is highlighted. What you want to achieve is a single button action that perform the functions, without worry about which window is in focus or highlighted. One possible solution may be to map windows media keyboard keys to it. The standard ones applicable are Vol+, Vol- and Mute. Then you can use any off the shelf compatible keyboards or remotes. Other possibilities include bluetooth remote, or IR receiver, or a network API for integration or associated app.
As for my earlier question on volume control steps, it's not working for me on the current UI. I have set the "Slider" and "Value display" to 1dB. But when I click on the slider bar, it still change to any value at 0.1dB increment. Arrow keys also increment/decrement at 0.1dB steps regardless of the settings.