I love bringing several technologies together for a new purpose and there's no better purpose than Halloween. I searched a little and didn't see this method described, sorry if it's been posted before. Thought it would benefit the larger haunt community.

Basic idea: Smartphone > bluetooth to FM radio adapter > old radios

I use my bluetooth to FM radio adapter every day in my car for handsfree operation and for playing music. Works great. I just had to use this thing for Halloween. My thinking was that old radios can be cheap, small and run on batteries so they can be hidden anywhere in your haunt without the need for cords, etc.

Any smartphone (Android, iPhone, Blackberry, etc.) that can broadcast A2DP audio can work. You can also use a PC (do iPads have bluetooth? Not sure. If so, use yours!).

Steps
  1. Pair your phone to the FM adapter
  2. Tune your radios to the right station
  3. Play the sounds

I'm using a Motorola MOTOROKR T505 (online for $59 now), an old Windows Mobile Phone (got a new Android phone now) and two CD player "boom boxes" with FM radios that we had around the house. Now I'm in control of sound effects all over my yard from my handheld. One catch is the on my unit the FM range isn't very far. If you're buying a bluetooth to FM transmitter to try this, get one with the longest possible FM range. My bluetooth range is longer than the FM range on this.

This year I'm going to try something new. I'd like to have the same set up described above but create my sound effects on only one stereo channel. My radios' balance will be tuned to different channels, either left or right so that I can control which radio plays the sound.