The idea is to control some LEDs with the music.
Wikipedia:
A light organ is an electronic device which automatically converts an audio signal (such as music) into rhythmic light effects.
It looks like it is not so easy to split the sound internally so that it goes to the add-on board or speaker AND can also be processed to control LEDs depending on the music.
But the idea is:
Send the audio signal to the add-on board or speaker directly AND process it also internally with a FFT to find out if the sound is in a certain frequency band.
It should be possible to set if ON is low or high because there are setups where the LEDs turn on when the pin is low.
Ideally one could set the number of channels (1-8) and the lower and upper frequency of each band.
The assignment of the band to a pin should be selectable.
For more ideas how to fade the LEDs and how to implement a simple peak hold, check the web page below (in german)
GPU_FFT might be usefull for that
This one works quite well for an Atmega (german):
mikrocontroller.net/topic/131525