I might suggest starting with something like an Arduino Uno. This will have some examples out there such as the blink example that shows how to blink an LED. Depending on the LED you are using you may need to run the signal through a relay as the Arduino will not run 12V.
If there is only one phase it may even be simple enough to program the lights to blink in time with the sound and simply trigger the Arduino in the same way your motion detector triggers the playback.