Sound when no one is in the room

I know this is the hardest thing to do with arduino (to make sound when no ones is in the room), but I´ll give it a try on this forum because you really helped me before. Im a fine art student and Im going to exhibit a sound piece in an art museum room. My idea is to make the creaking and crackling noise from the tree floor, but I only want it to sound when the room is empty. If I understand it right this could only be done by a camera sensor(?), but do anyone have some other ideas?

Thankful for any advice!

/Johan

You could use a IR sensor like this:

They have a pretty good range, and width. Maybe use 2 or more to check multiple corners/rooms.
Furthermore, you can adjust the sensitivity, and they give you a digital signal, so they are easy to program via the Arduino.

Like @krakert said, that is a good choice, search for PIR sensor on your favorite retailer's site. The model he's shown has a better range than the smaller models.
How many entrances does the room have?
What are the dimensions of the room?

Ok, but can pir sensors be coded so they send a signal when the room is empty? Couse thats when I want it to turn the sound piece on. I dont know the rooms size exactly but I guess around 5x5 meters. And only one entrance to it.

If you use them with some MCU (Arduino?), you can take care of that in your code.
Count some seconds after the last signal from the PIR sensor, then start making noise. If you receive another impulse, stop the sound and begin counting again.