Using soft serial for MP3 Trigger.

mp3trigger_softserial.ino

#include <SoftwareSerial.h>

SoftwareSerial sSerial(10, 11); // RX, TX

void setup() {
  Serial.begin(9600);
  while (!Serial);

  // set the data rate for the SoftwareSerial port
  sSerial.begin(38400);

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

void loop() {
  // Play track 1
  sSerial.write('T');
  sSerial.print(1);
  delay(2000);
}