The code to synchronize an LED bar to music using an Arduino.

Arduino Music Lights v1

//Arduino Music Lights v1
//by paplaukias
//
//uses the FFT library which can be found on http://arduino.cc/forum/index.php/topic,38153.0.html
//
//[email protected]
//http://www.paplaukias.co.uk
 
#include <fix_fft.h>
//set AnalogPin for audio input
#define AUDIOPIN 0
//set the sensitivity for the bars
#define HIGHEST 5
 
char im[128], data[128];
char data_avgs[8];
int i=0, val, bass;
 
//shift register pins
int latchPin = 3;
int clockPin = 4;
int dataPin = 5;

//explicitly set the LED output
int level[9] = {0b00000000, 0b00000001, 0b00000011, 0b00000111, 0b00001111, 0b00011111, 0b00111111, 0b01111111, 0b11111111};
 
void setup(){
  //set pins to output so you can control the shift register
  pinMode(latchPin, OUTPUT);
  pinMode(clockPin, OUTPUT);
  pinMode(dataPin, OUTPUT);
  Serial.begin(9600);
}
 
void loop()
{
//read data from the audio input and store it in an array
for (i=0; i < 128; i++){
  val = analogRead(AUDIOPIN);                                  
  data[i] = val;                                     
  im[i] = 0;                                                   
  };
 
//use the FFT to convert audio data from
//a voltage domain to a frequency domain
  fix_fft(data, im, 7, 0);
 
  for (i=0; i<64; i++){                                    
  data[i] = sqrt(data[i] * data[i] + im[i] * im[i]);  // this gets the absolute value of the values in the
      				                                            //array, so we're only dealing with positive numbers
};   
 
//average a couple of frequencies together
data_avgs[0] = data[0] + data[1];	//average together
data_avgs[0] = map(data_avgs[0], 0, HIGHEST, 0, 8);	//remap values for the LED bar
    
//check that the value doesn't exceed 8
while(data_avgs[0] > 8){
data_avgs[0] = data_avgs[0] - 8; 
}
    
int bass = data_avgs[0];
    
//send the data you want to display to the shift register
shiftOut(dataPin, clockPin, MSBFIRST, level[bass]);
//output the data written to the shift register
digitalWrite(latchPin, HIGH);
digitalWrite(latchPin, LOW);
delay(15);
}