There's no way to reduce volume in Quest, and (sorry) I don't know any editors. (Why don't you search in Google?)
However, every problem has its own solution.
SOLUTION: Reduce the computer volume, or better still, make a command to stop (and play) that blasting sound.
Here's how to make the command:
If [object] [player] has flag [music]
Then
Stop sound
Unset [object] [player] flag [music]
Else
Play sound [mymusic]
Set [object] [player] flag [music]