Apart from playing audio recordings. How would you do that on an arduino?
Though it’s not terribly complicated on a RPi – the OS doesn’t really get in the way for doing simple things like this, which you could as suggested above pretty much do in Scratch which is likely to be easier to programme than anything available for an Arduino.
You can connect a PIR directly to GPIO on a RPi, which is what I have in my skull (which also flashes LEDs and plays audio recordings). PIR is in the nose and pretty much everything directly connected to GPIO:
[video]http://vimeo.com/144826651[/video]