EFX-TEK just came out with a new board, the AP-16+ which was designed for use as what your describing.
* Plays 16-bit, stereo WAV files; any bit rate up to 44.1kHz
-- no MP3 encoding required
* Files are stored on SD card
* Can loop background track ("ambient.wav") without user intervention (other than selecting loop mode)
* Can play one of 16 files using play input (button, dry contact, DC input, or PIR) or one of eight auxiliary files from parallel inputs
* Can randomize one to 16 files from start input
-- all files played before repeats
-- max files based on select switch
* Manual operation overrides background (ambient) loop
-- at conclusion of file ambient loop will restart if enabled
* Relay output active when file plays (not active for ambient loop)
* Pots to set pre-play and post-play delays
* TTL serial connection for external control from Prop-1, Prop-2, Prop-SX and other controllers
You can hook up a light or other device to the relay, trigger it with just about anything, and I know the on board audio amplifiers will just about blow a 60w speaker.
I belive these will be available with in the next two weeks. I will be ordering at least a dozen or so for our haunted attraction.