Well, you could probably do it with just the ATMega on board the Arduino. The following are my recommendations
For detecting people:
or
For playing sound:
For the motor (if it's too big, which it most likely is, to power via the Arduino):
Transistors and External power supply
I know all the things are a bit scattered, but hopefully that would give you a general idea about how I'd do it. 
PS: Using the Ultrasonic sensor, you could easily program the Motor and sound to get more frantic by reading how far an object is.
PPS: Please keep in mind you may run out of I/O pins during this project! I don't know which Arduino you have, but you may need the Mega or even multiple Arduinos (or multiplexing, but I'm a noob at that :P)