You know I was just thinking today, I need to post a neat discovery that would work great for this. I try to offer any awsome tips on here as they have let me plug my voice over services.
As some of the posts above mentioned, the easiest and best way to do it, is with the pica boos, and with the amp incuded, it does everything with 1 little box, you can get this for under 150 with the step pad. Works everytime flawlessly.
Here is what I found. You can use the motion sensor. I have been getting those 4 outlest boxes and putting the motion head in the top hole and a power cord out the bottom. That way you are more complient with the codes as all your connections are inside the box. Or you can use the screw in socket hooked into a power strip. You then can hook lights to it. but how do you get the sound to start and go off? Well you get an mp3 player/cd/tape and either hook a relay between the audio feed to your amp, or this real cool and easy thing I found at Spencers is what they call a party to go box I think. It is like a guitar amp, but for an ipod. It has the amp and speaker together in 1 box, about 15 watts, but plenty loud. They had an original price of around 130 bucks on these...ouch, but they were clearing them out for about 40 bucks each. There were 3 left, I got 2 and the guy said if I bought the last one, he would sell them for 30 bucks each, so I got all 3. I tried using a guitar amp to power up, but it took it like 3 seconds to power up, but these babys power right up. So you would have the mp3 continously sending sound to this thing looped and then the sound just comes on and off with everything on the power strip that is pluged into the motion sensor. No special wiring or anything. The only downfall is the sound will not start at a certain point. When the amp is turned on it just turns the sound on in progress. This works fine for something like a scream over and over, or in my case the sound off a car reving and horn over and over, or my electric chair, which is just electric sounds and screaming, so when it kicks in it really does not matter. But if it is something saying dialog, or lets say a scare sound like a train horn that needs to start at the beginning it might not work for you. But a monster roar repeating would probably work. Sorry for typos and spelling. I am lazy and it is late. Hope you get something out of this. There are some tuturials on hacking the motion lights on here somewhere.