Sorry to bump an older topic like this, but I came up with an easy, cheap and dirty hack that might interest you. Coincidentally, motion activated sound effects were what brought me here in the first place!
I wired a security light to run off an extension cord, and mounted it in the ceiling of my garage on a sheet of plywood. No need to open up the electronics even, just wire the cord directly to the leads. Then I put screw-in plug adapters into each of the light sockets. When the motion sensor is tripped, it sends electricity into two devices. (I know this has been covered elsewhere, just covering my bases by being as clear as possible)
The first plug was hooked to a strobe light, pointed at a chainsaw hanging from the wall. The second plug was hooked into a cheap pair of externally-powered computer speakers. The audio plug from them was hooked into the headphone jack of an MP3 player, which had a 10-second clip of a chainsaw sound effect playing on loop.
Basically, the sound effect was always playing, but the speakers weren't on until the motion sensor was tripped. The whole rig was put on the plywood as well (for safety), and covered with a scrim (except for the motion sensor, obviously).
It's not as pretty as building a custom circuit, but for ~$30 total it worked pretty well, and I got a lot of complements. I already had the MP3 player and the extension cords though, so your cost may vary. It has the added benefit that it can easily be disassembled and fitted with different effects.
