This is a basic example of playing five short MP3s in a loop.

mp3-trigger-basic.ino

void setup() {
  // Start the serial port at 38.4K
  Serial.begin( 38400 );

  // Set volume
  Serial.print( "v" );
  Serial.write( 0 ); // 0 = maximum volume, 255 = minimum volume
}

void loop() {
  // Loop from 1 to 5
  for ( int i = 1; i <= 5; i++ ) {
    // Play file i ( 1 to 5 )
    Serial.print( "p" );
    Serial.write( i );

    // Wait a bit
    delay( 1000 );
  }
}