Wait...what? I think I'm misreading this.. Did you actually give access to master volume settings to scripts? ...If so, that is a really bad idea and needs to be fixed. Guaranteed 100% that any scripts using those will not be supported in the future.
I use sound APIs that have channels built-in, and it works well, so I'd be inclined to create additional data for playing sounds that the sound engine can manage separate of user settings, although this is a lot of work and the allegro sound stuff is not fun. Saffith's suggestion sounds pretty good to me, I'd go with that