MUS

The MUS format is a slightly altered MIDI format 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.

The later Vanilla Doom versions added support for MIDI files, which meant WAD authors no longer needed to convert MIDI files to MUS.

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.