Apparently, you can get tiny little MP3 players for less than $5 now. So, there's no reason not to grab one and start hacking it up, taking out the essential parts, and putting it into a much cooler case - like this cassette tape. Instructable-r BrittLiv used a $4.26 LEGO-style MP3 player, added an SD card, and managed to rework the reels to serve as push buttons to control the music.
The eventual inside wiring looks a little complicated, but its actually very straight forward, and should be easy enough for anyone who can use a soldering iron:
Check out the full how-to on Britt's Instructable: Cassette MP3 Player