The MintyMP3 hardware is a standard mix of high power 8-bit microcontroller and ASICs.
http://www.ladyada.net/make/minty/
The project combines an Apple iPod with a T&A-hifi stereo system. iPod is plugged into the stereo and deliver the Audio-signal to the T&A-system and it is remote controllable with the T&A remote control without an own infrared receiver.
http://ipodlinux.org/IPod_to_T%26A_remotecontrol_adapter
Chu Moy designed a very popular headphone amplifier that's easy to build, and it can be built small enough to fit in a pocket, power supply and all. It's powerful enough to drive very inefficient headphones to thunderous volumes from even weak sources, and it sounds excellent considering that you can build for just US$20.
http://tangentsoft.net/audio/cmoy-tutorial/
MP3 hardware decoder connecterd to PC parallel port
http://www.heise.de/ct/ftp/projekte/mp3player_1/default.shtml
information on fitting car with a MP3 player wich runs on Linux
http://utter.chaos.org.uk/~altman/mp3mobile/
http://www.aaroncake.net/circuits/cdsupply.htm