Hi, There are boards that have an SD card to store MP3s and can be controlled by Arduino. Examples:
http://www.ebay.com/itm/Arduino-VS1003-SD-Card-Player-Support-MP3-WAV-Midi-/160639650391?pt=LH_DefaultDomain_0&hash=item2566de8a57
And this one, with Library:
http://www.ebay.com/itm/USB-SD-MP3-Shield-With-64Mbit-Flash-And-Audio-Amplifier-Arduino-Compatible-/280924908139
And Adafruit has one too I think, as a kit....