by Jose Daniel Herrera @ arduino-cool.blogspot.com:
I present this new Arduino project: A full MP3 player based on Arduino.
As you can see in the photo, the idea was to build a standalone player, in the style of antique radios or ‘tapes’, in the age of Iphones ….
The player uses a small module based on the VS1002d chip (now discontinued, but who had a drawer). The module in question I bought at Futurlec . Currently you can get similar modules on ebay for a bit more than 10$, although the chip is the VS1003 o VS1053. You can also use a SparkFun MP3 shield, but significantly more expensive (this shield already includes an SD slot).
The three chips are quite compatible in terms of programming, and indeed almost all initialization the I have taken from the library to the SparkFun MP3 Shield
Arduino standalone MP3 player - [Link]