More modules should be used for Doom maps!
Modules are basically MIDIs, but with their own samples bundled in, meaning they are often less than a megabyte in size and don't bloat your wad's file size.
They have been used for games such as Unreal, Deus Ex, and Jazz Jackrabbit, and can easily fit into Doom.
They only work in DSDA-Doom, ZDoom source ports, and Edge-classic however, so if you're looking to make maps for vanilla, Crispy, or some obscure port, the music will not work.
There is a useful site called The Mod Archive with its huge library, and a handy online music player for instant listening.
Don't know where to start? Here are some suggestions:
Scroll down in the home page to see the newest modules, favorited modules and some randomized picks.
I think Legacy and Doomsday engine use module files as well iirc, as well as Eternity engine... And maybe Delphi Doom too?
I have switched to routinely use modules for my boom/mbf21 Doom maps, the range of sounds achievable is insane and there is a lot of untapped music available. Imagine your map not having Plasma or You Suck...