Volumouse

Volumouse provides you 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. For example: You can
configure Volumouse to use your mouse wheel for volume control when the
Alt key is hold down, when the left mouse button is down, when the mouse
cursor is over the taskbar, and so on...
When the conditions that you define are not satisfied, your mouse wheel
will be used for the regular scrolling tasks, exactly as before.
http://www.nirsoft.net/utils/volumouse.html

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