GMWinVolume

GMWinVolume is a Windows only GameMaker extension that allows you to control the global system volume. This isn't the volume specific to your game and assets, but the operating system level volume.

• os_get_volume() - Returns the current system volume between 0 - 1
• os_set_volume(volume) - Sets the current system volume to the percentage you provide (a number between 0 - 1)

This extension is basically a wrapper for the solution found here.