In the German computer magazine CT issue 9/1999 p. 200 and 10/1999 p. 260 I found an article about turning an old PC (from 286 upward) into a MP3 player, by using a MP3-Hardware-Decoder at the parallel port. Authors Homepag .
Also mentioned there, is a software by Klaus Peichl, which doesn't need a hardware decoder.
Though both programs are DOS based, I mention them here. I hadn't time to look for an according Linux solution.
Cajun is a program that allows you to turn any computer (>75mhz) into a massive audio jukebox for your car or home. It uses the matrix-orbital serial display and supports the IRman infra-red remote control interface. Soundcard output is delivered to your (car or home) stereo for amplification. The software supports a hotlist and shuffle mode. It includes FM/Video4Linux support, icecast/shoutcast support, CrystalFontz serial display support, and choice of mpg123 or xaudio for driving the sound card.