Adding blinking lights to anything is a surefire way to make it totally obnoxious. For some of you, that's a good thing.
Here are two techniques you can use to add sound-reactive lights to your stereo, boom box, computer, or pretty much anything that makes music.
The first option is to use something like an EL Wire Sound Kit. These are sold online from various places and include some combination of electroluminescent wire, a power adapter, and a special power inverter that drives the wire's power in response to sound picked up by an integrated microphone.
The whole kit typically costs around $50. You can get the EL wire in different lengths, colors, and thickness. You can use splitter cables to drive multiple strands of wire. It's a great, uncomplicated solution, but it does have some drawbacks.
One problem with the EL Wire Sound Kit is that EL wire makes a high-pitched sound when it turns on. Loud music will drown it out, but it's no good for Kenny G. Also, the effect is pretty much restricted to blinking on and off.
For a more advanced option, try the Apollo Jammer LED Music Controller by Elemental LED. It's $99 and comes with a strip of RGB lighting that you can upgrade to any length you want.
Unlike the EL wire kit, you can route your music directly into the Apollo Jammer instead of relying on a microphone. You plug your audio source into the minijack input on the side, then run the included minijack cable out from the Apollo Jammer to the aux input on your stereo.
Once you're hooked up, you can adjust the sensitivity of the effect with a button located on the top of the device. A nearby button allows you to select from one of the 10 lighting modes. Some modes use different colors, some respond to bass or treble, some are constant, and some blink. You're bound to find something you like.
Another advantage of this system is that it works with all kinds of RGB LED products, including light pucks that you can mount on a wall behind your stereo. If you want to really go nuts, there's a DMX connection on the side that can interface with a huge range of professional lighting equipment.
The only real downside to the Apollo Jammer approach is that the RGB LED strips and products it works with are still pretty pricey. Tape strips of RGB LEDs run about $20 per foot, and club-quality DMX lighting products are typically priced in the hundreds.
So there you have it, two effective options for creating a light show with your stereo. Go crazy.