Simple example of playing random tracks.

MP3random.ino

// libraries
#include <SPI.h>
#include <SdFat.h>
#include <SdFatUtil.h>
#include <SFEMP3Shield.h>

#define NUMBER_OF_TRACKS 5

// initialize MP3 card
SdFat sd;
SFEMP3Shield MP3player;
int lastTrack;

// setup
void setup() {

  if(!sd.begin(9, SPI_HALF_SPEED)) sd.initErrorHalt();
  if (!sd.chdir("/")) sd.errorHalt("sd.chdir");
  MP3player.begin(); // start mp3 player shield
  MP3player.setVolume(10, 10);
  randomSeed (millis()); // set a more random seed for the random function
  // Note the seed is after the MP3 init, and the SdCard is asynchronous 
}

// loop
void loop() {
  int randNumber;
  
  randNumber = lastTrack; // just to get into the next loop
  while(randNumber == lastTrack) // keep choosing if it was repeat
  {
    randNumber = random(NUMBER_OF_TRACKS);
  }
  MP3player.playTrack(randNumber); // range of tracks to randomly play.
  lastTrack = randNumber;
  while (MP3player.isPlaying());
  MP3player.stopTrack();// stop track
}