Need a trip wire or door switch to start music

[kg4ozx]

We are trying to play a little prank on someone, and we need some way to start music (CD or MP3 from computer or a player) when he opens the door to his office.

I was thinking surely there would be some kind of toy laser tripwire or something that does this. I've searched ThinkGeek to no avail.

Any ideas would be appreciated!

Thanks!