Volumouse

HERE

Volumouse provides a quick and easy way to control the sound volume on your system — simply by rolling the wheel of your wheel mouse. It allows you to define a set of rules for determining when the wheel will be used for changing the sound volume.
There are much more options.
I just installed it, and it works nicely.

I have been using Volumouse for a few months now and it works well for me.
I have it set up for changing my volume and window transparency.

If you want a custom step size, you can type in any number you want instead of selecting one from the drop-down menu.
For example, I typed in 100 for my volume step size because I wanted a finer adjustment the 500.