IMF (probably) stands for "ID Software Music Format" and is remarkably similar to the Rdos RAW file format. It contains a list of the data bytes to be sent to the OPL chip on an Adlib or SoundBlaster sound card, as well as how long a delay should be left between sending the bytes.
The end result is a very lightweight music player, which uses very little CPU time - essential in the days when games were required to run smoothly on a 286.
A complete technical description of the format can be found in the ModdingWiki article on the IMF Format.
- IMF2MIDI can be used to convert IMF files into MIDI files
- Adlib-XMMS - a free Adlib player that will play IMF, CMF and DRO files, amongst others (XMMS plug-in source code available)
- AdPlug - a free Adlib player that will play IMF files (DOS, Winamp and XMMS versions available)
- Wikipedia:IMF_(music) - Wikipedia article on the IMF format
- ModdingWiki:IMF Format - technical documentation