Just set a cron on a coworkers machine to play "What does the fox say" at max volume at 8 when he's the only one here.

May need to review the security footage in the morning.

YouTube link please! That's hilarious

This is evil genius stuff here. Of course if someone did this to me and I found out who it was...

I think I'd have to step it up a notch and have the computers use the tts engines to reveal personal details and begin to discuss among one another the merits and failures of the coworker.

Starting slowly with one machine, quietly saying

"psst hey, 192.168.42.77, you see the garbage that frank has been working on?"

And have the volume start quiet, audible, but barely, and increase in volume until it's a rabble of voices which suddenly stop mid sentence, as if it never happened at all.