Run sndvol32.exe.
You have 2 interesting volumes: The master volume (leftmost column), and the "Wave" volume.

Play some wave file.
The volume at which you hear the sound depends on both the master and the wave.

What is the dependency?
Is it simply output = wave * master, or something more complicated?

Or, if output = f(wave, master), then how do you convert:
f(wave, master) = f(100%, what?)
f(wave, master) = f(what?, 100%)

Obviously f is a continuous function and the set of values 0 <= f(x,y) <= 1 is mappable to 0 <= f(x, 1) <= 1 and 0 <= f(1, y) <= 1 using some mapping.