Its pretty simple. I took the flashing circuit for an led box and built it into a ready made 2.1 system, then made a new housing for it.
Have a look here for a video of the finished product:http://youtu.be/0kvhXYVkGGU
Credit to motadacruz for his instructable as it was his awesome guide that got me doing this one, anyway have a look at this for the circuit but forget about the stereo cable as we are going to hard wire this into our audio circuit
The Circuit to control the LEDs consists of a NPN transistor (everyone recommends tip31 but i used tip 121 as thats what we had in the electronics cupboard). The base (left leg) connects to the signal source i.e the positive wire on the speaker. The collector (middle leg) connects to the first negative leg in the chain of LEDs (the amount of LEDs you can run depends on your voltage) the emitter connects to the 0v rail on your power supply. Thats it. there is no need to ground the negative audio signal like everyone else states.