All of these settings can and should be changed in the in-game Settings > Audio menu.
fAudioMasterVolume sets the master volume level. It is changeable in the in-game Settings > Audio menu as Master Volume. It has a soft cap at 1.
Default is 1.
See the beginning of this section. This sets the volume of this particular section of sound. It has a soft cap at 1.
See the beginning of this section. This identifies the sound group to be modified in the corresponding fVal#.