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(!!)
libTiMidity is a MIDI to WAVE converter library that uses Gravis Ultrasound-compatible patch files to generate digital audio data from General MIDI files. This library based on the TiMidity decoder from SDL_sound library.
Schism Tracker is a music editor that aims to match the look and feel of Impulse Tracker as closely as possible. This SourceForge project page is more or less abandoned -- current development is ongoing at schismtracker.org.
Remember when you wrote your games in Pascal? Well continue that fine tradition by using JEDI-SDL! Which works with Delphi, Kylix, Free Pascal, Gnu Pascal and TMT Pascal. Cross-platform game develpment has never been easier!