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.