Volume Control supports the scrolling by mouse wheel. This works fine if the volume control is inside menu. But this not works if the volume control is on the toolbar. By default the mouse wheel scrolls only the active window, so you cannot to scroll something in other window.
This can be fixed with special drivers of 3-rd party software. I can recommend the
Tordex Wheel. With Tordex Wheel you can scroll the window under the mouse cursor, no matter what window is active. And you can scroll the Volume Control.