Adafruit "Music Maker" MP3 Shield for Arduino w/3W Stereo Amp - v1.0 features the VS1053, an encoding/decoding (codec) chip that can decode a wide variety of audio formats such as MP3, AAC, Ogg Vorbis, WMA, MIDI, FLAC, WAV (PCM and ADPCM). It can also be used to record audio in both PCM (WAV) and compressed Ogg Vorbis. You can do all sorts of stuff with the audio as well such as adjusting bass, treble, and volume digitally.
All this functionality is implemented in a light-weight SPI interface so that any Arduino can play audio from an SD card. There's also a special MIDI mode that you can boot the chip into that will read 'classic' 31250Kbaud MIDI data from an Arduino pin and act like a synth/drum machine - there are dozens of built-in drum and sample effects! But the chip is a pain to solder, and needs a lot of extras. That's why we spun up the best shield, perfect for use with any Arduino Uno, Leonardo or Mega.
Adafruit "Music Maker" MP3 Shield for Arduino w/3W Stereo Amp - v1.0 includes an onboard 3W/channel stereo audio amplifier that can drive 4 or 8 ohm speakers.
Adafruit Website: https://www.adafruit.com/products/1788