MDfly makes some mp3 players that can be controlled with a key press or with a serial interface. The serial interface command can select a track, pause, play, and the usual stuff. The only rub is that the mp3 files have to be named with a number like 01.mp3.
So to get your complete solution, you just need an Arduino, a 12 key keypad, an mp3 player card, a SD card with the audio files, a case to put it in, and the software to make it work. Clearly the trick bit is writing the software.