MUS

The MUS format is a slightly altered MIDI format used in the Doom engine soundtracks. MUSes are nearly identical to MIDIs, only slightly more compact.

MUSes have a size limit of 65536 bytes.

The later vanilla Doom versions added support for MIDI files, which meant WAD authors did not need to convert MIDIs to MUSes anymore.

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.