Mikmod is a module player and library supporting many formats, including mod, s3m, it, and xm. Originally a player for MS-DOS, MikMod has been ported to other platforms, such as Unix, Macintosh, BeOS, and Java(!!)
AdPlug is a cross-platform OPL2 audio player library. AdPlug plays music, originally composed for the AdLib (OPL2) audio board, on top of an OPL2 emulator or by using the real hardware. There are multiple player software interfaces available.
J Sound System aims to become a multi-platform, API portable sound system for Free Pascal programs written in object-oriented Pascal and i386 assembler.