Our desktop techs are always pranking each other here where I work.
Some of them have their desks back to back like you see in the detective shows. So they put a "Y" splitter from one of their PC's to the other guy's speakers, then they played random sounds like burps and farts, and the ever popular:
"WOO HOO! HEY EVERYBODY! LOOK AT ME! IM WATCHING PORNO OVER HERE! WOO HOO!" sound clip.
They all had a good laugh over it, and then the victim went to work dilligently searching his PC for the hidden utility, trojan, virus etc. they used to do it to him. Of course, he never found it. He even formatted his PC, and they started playing sounds the very first time his newly formatted PC booted up to the Windows desktop.
He never thought to check his speaker wires.