Well, I wrote a plugin for questions like that:
Full source(in visual C++). Shows how to get the spectrum info
from winamp and display it. You could just hook that code and send it to your own! For something like LED's, I would recomend using analog
controls for them(modulate the brightness). If that's not possible, then filter the spectrum down to the number of led's you have, boost the higher ends of the spectrum, and watch for a major change in the average spectrum energy.
Anyhoo. If you can write that code, the plugin will give you winamp+spectrum info
Hope this helps.