As you all might know, the music volume is controlled with a value in the range [0, 100]
Unfortunately, this sets the volume on a linear scale, whereas human ears hear the loudness on a logarithmic scale.
To set the volume of the music more naturally (meaning 50% feels like half as loud, etc), do this:
-> XAudio2: Set music volume to max(log10(myVolume) * 50, 0)
where myVolume is your desired percentage in the range [0, 100]