For around $30 in parts and a good amount of patience, you can have a completely open source and hackable mp3 player ready to go. It can be modified to accept serial commands, be embedded in an art project, used as the voice of your next smart talking robo-sidekick, or filled with music and used as is. Put in whatever size card you want, up to the theoretical limit of the MMC format.

The Daisy is a multipurpose sound player for embedded applications. It can be used as a standalone personal music player,as the sound for an art project, in a kiosk, as a museum tour guide, in a toy, or anywhere that high quality embedded audio is desired. It uses MMC or SD flash memory cards so storage size is unlimited. It has several interface modes for either human or machine control.