Use this following sketch to test the uMP3 using the serial port connection on the Arduino and Arduino-clones.

/**************************************************
* Simple pass-through serial application for
* Arduino and Arduino-clones.
* You can use this for testing and
* updating the Rogue Robotics
* uMMC Serial Data Module or
* uMP3 Playback Module.
*
* You will need the "NewSoftSerial" library
* available at the Arduino website.
* http://arduino.cc/
* http://arduino.cc/en/Reference/Libraries
***************************************************/

#include <NewSoftSerial.h>

// You can set this to whatever pins you have the uMMC or uMP3 connected.
// e.g. 4 is connected to uMMC "T", 5 is connected to uMMC "R"
NewSoftSerial out(4, 5);

// If you are using this to update the firmware on the uMMC or uMP3,
// you will have to make sure that both the Serial connection and the
// out connection are set to 9600 bps.
void setup()
{
  out.begin(9600);
  Serial.begin(9600);
  pinMode(13, OUTPUT);
  digitalWrite(13, 0);
}

void loop()
{
  digitalWrite(13, 0);
  if(out.available())
  {
    digitalWrite(13, 1);
    Serial.print((uint8_t)out.read());
  }
  if(Serial.available())
  {
    digitalWrite(13, 1);
    out.print((uint8_t)Serial.read());
  }
}