"Fish Heads" cannot be the worst of all possible soundtracks, if only because one of the Barnes of Barnes & Barnes is actually Bill Mumy, a long time sci-fi genre actor. That's gotta count for something!
No, the absolute worst sound track of all time would be a record of one repeated sound bite - Jim Carrey in "Dumb and Dumber":
"You wanna hear the most annoying sound in the world? EHHHHHHHEHEHHHHHHHEHEHHHHHHHHHEHEH!"
I have friends who claim that's the noise elves make when they become confused....