Здесь находится подробная инструкция по сборке MP3 плейера на основе MMC/SD карт. Есть файлы схем, PCB и прошивки для PIC.

This is small MP3 player. It’s cheap and of excellent quality. An MMC has only seven pins and is currently the cheapest flash memory on the market! SD cards are now fully supported!
Powered by a single AAA (HR03) 1V2 Ni-Mh battery. Runs on a 18LF452 at 20 MhZ.
Up to 256 kBps bitrate at 44,1 kHz supported. VBR may peak at 320 kBps. ID3 v2.3 tags supported.
Full user control (volume, track, skip) with a Micro joystick.
Real Time display of bitrate, sampling rate, elapsed time, songnumber, …
LCD contrast adjustable.
Voice Recording in PCM-format could be added later!