I've just built something like this. Someone told me that a "light organ" is what i should research, and i'm going to make one later on. They're more complex than what i've made. Now, mine doesn't brighten on higher frequencies, it's the opposite (because of the basic design of the circuit) where the low tones (especially drum kicks) will pulse the light. This uses an LED and 5v DC but, depending on the specs of your transistor, it can be modified without harming your audio source. I've considered hooking it up to my car's 12v and pulsing the parking lights, but that might be a little much for such a little transistor.
This works very well for me, but it might be because i push the audio output to the point of "clipping"
Here's the basic circuit:
**broken link removed**
It can easily be doubled to handle both channels of a stereo input.
I am just a beginner in electronics, so there will probably be better schematics and advice from other people.