Luci: Grow up already. you're 18 for God's sake.
Skyman: I don't consider waisting both public tax dollars and our valuable firefighter's time a joke. (Plus making a false report like that is a crime.) Also, the other things you listed are vehicular vandalism, also petty misdemeanors.
VirgoChicK: Tetanus is not a laughing matter.
Low-Key: that is just disgusting. What would be funny though is that while you were attempting to do what you describe if your foot slid off the commode, went in, and you fell over and everyone then heard you cursing. That could be funny.
The only practical joke that might be worth trying would be to use an IPOD with the FM broadcaster attachment and see if you could get ingress into the sound system OR
If your congregation uses the old tapes still, to take one of your own that looks just like the ones they use, record your own special message, and then try to switch them out. (Or burn a CD if they use CDs now) If yo know the song, it would be funny to let the first stanza play as normal and then fade in your modification or message.
(It would be even better if you could somehow manage to stick the off and eject and power buttons so that they would have trouble turning it off for a while.)