SUMMARY
I've been using Volumouse on windows since forever, but i can't find any reliable alternative for linux. It would be nice if you could implement this into kde.

What is Volumouse? It's a program that helps you adjust the volume using the mouse wheel. There is also an alternative to it with more features, called Volume2.

You can use both to assign a key or button and when you keep it press and spin the mouse wheel the volume changes, no matter where the mouse pointer is or in what app you are.
This is very useful to easily adjust the volume, i personally use Right Mouse button + Mouse wheel, because is the best option, I always have my hand on the mouse so i can instantly adjust the volume anytime I need / want.

I think it shouldn't bee to hard to implement this since part of it is already implemented in kde, you can change volume with cursor on volume icon and using mouse wheel, all it needs is to add an option to do the same when you keep a button pressed, instead of holding mouse on volume icon.