NPC Code:
$pref::graal::midivolume
$pref::graal::mp3volume
$pref::graal::sfxvolume
$pref::graal::radiovolume
You could try modifying these, but they effect all sounds, not just that one sound. I was playing the option variables, but I found that some of them didn't apply until I restarted Graal.
If you don't want to modify the volume of all sounds, I suggest using the setvolume function.