MUSINFO is a control lump originating from Risen3D and also supported by PrBoom+, ZDoom and GZDoom. It is used to associate music lumps to numbers, which are then referenced by "music changer" things with an editor number in the 14100—14164 range.
The syntax is as follows:
<MAPNAME> <number> <MUSICLUMP> <number> <MUSICLUMP> ... <MAPNAME> <number> <MUSICLUMP> <number> <MUSICLUMP> ... etc.
MAP01 1 D_ROMERO 2 D_STALKS MAP02 1 D_RUNNIN 7 D_MESSAG
Valid numbers for music lumps are in the 1—64 range, the value 0 is reserved for the map's default music. Within a map, each song is associated to music changers with an editor number equal to 14100 + number. So in the above example, a thing 14107 in MAP02 would trigger a music change to D_MESSAG, and a thing 14100 would trigger a return to the default music for the level.
Music changer things are triggered when a player enters the sector in which they are placed.