LMMS

LMMS (originally Linux MultiMedia Studio) is an open-source cross-platform music program. The .mmp files are normally-saved music files (in an XML-based format), while .mmpz files are compressed in zlib format (with a four-byte header pre-pended).

Links

 * Official site
 * Sourceforge
 * Wikipedia article
 * Official wiki
 * Instructional video
 * Discussion of decompressing compressed files
 * Discussion of reading files between LMMS versions