Fortunately, Hong Kong has mastered ultra cheap MP3, and with a little hacking you can add it to your project for around $3.00.
I'll demonstrate the hack using a Propeller, but the best part is that ANY micro-controller can use the device once you've got the project finished.
A generic MiniClip MP3 player (Source Ebay prices range from $0.26 -- $3.00)
Your favorite microcontroller. (I'll be using Propeller Platform, but any will do fine.)
A hot glue gun
A steady hand, and basic soldering tools.
Step 1: Teardown
The whole thing should slid out with a little prying.
Notice the little battery, as well as red/black wires. The Miniclip runs great a both 3.3v and 5v.
Step 1 > Carefully, desolder and set the battery aside, we'll use part of it later.
Step 2: Replace the power wires
Recommended > Add some hot glue to strengthen the connections once you've soldered them in place.
I used a couple breadboard wires.