9 years ago
Same way you do for incandescents. Take the audio input, run it through filters to seperate it into however many bands you want to control, run those separate signals through a driver that puts out the appropriate zero-to-maximum-drive-voltage range for the LEDs (and their load resistors, if you're using load resistors), bake at 350 degrees for one hour and apply frosting as desired.
You might want to add a stage which integrates those signals to extract a volume envelope from them before the final driver stage, so the LEDs output more brightly rather than flickering at audio frequencies,
Basically, if you start with the circuit diagram for a color organ, and replace the output triacs with something more appropriate to controlling the proper levels of DC, that should more-or-less do the job.
Of course these days another alternative would be to do all the signal processing in software, eg running in a microcontroller. Comes out to essentially the same thing, just a different way of getting there.