SparkFun MP3 Player Shield- MP3 decoder- Turn Any Compatible with Arduino Board into a Fully Functional MP3 Player- 3.5mm Audio Out Jack- 0.1" Spaced Header for Speaker Out- microSD Card Slot: Amazon.com:
MP3 Player Shield Hookup - SparkFun Learn
Amazon.com: SparkFun MP3 Trigger - 18 Trigger inputs to Alternate Functions Random & sequential Track Selection Transport Controls Even Volume up/Down Real-Time Volume Control Full-Duplex Serial Control : Electronics