Playing a little with the SDK I found these useful functions: waveOutGetVolume and waveOutSetVolume that read and set the volume of the wave audio device. They worked well while playing a CD on the computer: maybe can help you in your application.
Take a look at the attached project. It works on my machine that has only one audio device: in case od more complex configurations you'll have to struggle a bit to intercept the proper device: the waveOutGetNumDevs function can help you in finding how many devices are installed in your computer.
Roberto