MUS

The MUS format is a slightly altered MIDI format, created for DMX and used in the Doom engine soundtracks. MUS files are nearly identical to MIDI files, only slightly more compact. The file format has a size limit of 65536 bytes.

Support for standard MIDI files in vanilla Doom was added in version 1.5, which meant WAD authors no longer needed to convert MIDI files to MUS. However, since this was achieved by the game running MIDI2MUS internally when needed, authors looking for vanilla compatibility should make sure that the MIDI file they include can be successfully converted.

Limitations
The MUS format can only use the first 9 MIDI channels using the first 8 for music instruments and the 9th for drums. The reason this is forced upon the format is that older soundcards had an FM sound chip with only 9 FM channels. Although some at the time had 18 channels, these were less common.