The MP3 Shield Have many Advantages such like Control by Serial UART interface or keys onboard , Have Build in Audio Amplifier 3W , and has On board 64MBit SPI-FLASH, which you can Save Your Audio file , and play it later .
This Shield can controlled from Arduino By Hardware Serial “Rx D0 , Tx D1 ” and By Software Serial ” RX D7 , Tx D8″ So at first You can Use Hardware serial , easy to use but you must remove the shield when you want to Upload new sketch .
You can change The Serial interface pin by solder tools , Just remove the solder from Upper 2 pads and solder a small bridge between the 2 pads below to active Software serial Pin . look to the image attached to see these 3 pads .
You can also Bridge amplifier control bridge so you can turn on or Off by solder Amplifier Control pin , so This pin attached to arduino D9 Pin , I didn’t use it in this experiment
You can also Solder Pin header on This shield “Look at the Picture” So You can connect Speakers to this shield.