Teuthis has several kits and parts available for making your own MP3 player.
The Super-Simple MP3 Player is based on the PIC 16LF877, with plenty of processor time to spare. There is a bare bones assembly version, and a new C version with more features and stability.
It uses compact flash cards up to over 100 gigabytes (once they exist) including microdrives. (right now they are about 8 gig). It’s open source.
And most importantly, it plays music!