Arduino Grade Crossing Sketch
Model Railroading fun with the Adafruit Wave Shield in 12 Steps:

On the Hardware Side:

1) Get an Arduino ('328-equipped model preferred)
2) Get an Adafruit Wave Shield for Arduino (http://www.adafruit.com/products/94)
3) Get a FAT16 formatted SDCard
4) Copy BELL.WAV to the SDCard
5) Insert SDCard into Wave Shield
6) Attach open collector detector circuits to A0 and A1 pins (GROUNDED means detector triggered)
7) Attach LED anode to 5V or 3.3V pin
8) Attach LED cathodes through resistors to A2 and A3. (For 5V, use 470 ohm. For 3.3V use 330 ohm)
9) Connect Speakers to Audio Jack of Wave Shield, Attach Power, and turn on Volume potentiometer!

On the Software Side:

10) Get the WaveHC Library and install it in your libraries folder (http://code.google.com/p/wavehc/)
11) Build and Download the GradeCrossing.ino sketch to your Arduino
12) Trigger the detector circuit and enjoy