The table above is to give you the idea of what kind of soundtrack you want to install.

1. extras_new.wad - an essential download for everything to work. Contains all music for the maps. The rest of files contains the prompts to use exact music and music for Title Screen, Intermission and Text. TNT Evilution is installed out of the box.
2. plutonia_o.wad or plutonia_h.wad - pick one for Plutonia MIDI pack installation;
3. nerve_o.wad or nerve_h.wad - pick one for No Rest For The Living MIDI pack installation;
4. masterlevels_h.wad - just one option for replacement of music in Master Levels;



It might look like this:
"C:\Program Files (x86)\Steam\steamapps\common\Ultimate Doom\rerelease"
If not Steam is installed in another directory, as long as you find Steam\steamapps you will find that folder. Notice that the remaster uses the same folder as previous Doom 1 releases, not new folder.
Inside open "rerelease" folder.

The original Steam Discussions Thread

Plutonia MIDI Pack Doom Wiki[doomwiki.org]

Peter Lawrences' (Megapshere) YouTube Channel

Master Levels for DOOM II 25th Anniversary MIDI Pack Doom Wiki[doomwiki.org]

Master Levels for DOOM II 25th Anniversary MIDI Pack Official YouTube Video * - Didn't downloaded it because I can't use bandcamp and downloading\coverting from Youtube would hurt quality of sound a lot.

Master Levels for DOOM II 25th Anniversary MIDI Pack Soundfont Mix

No Rest For The Living Community MIDI Pack Doom Wiki[doomwiki.org]

No Rest For The Living Community MIDI Pack Official YouTube Video

No Rest For The Living Community MIDI Pack Soundfont Mix

DAR's YouTube Channel Channel

DAR's TNT: Evilution Marathon

DAR's Plutonia MIDI Pack Marathon

There are 2 types of wads to consider searching for code and files.
You are familiar with IWADS, most certainly, but these are modified and contain UMAPINFO. If you have no idea what it means (suddenly) then it is explained later.
And a brand new extras.wad. Which contains a lot of important stuff to matter in this thread.

Inside extras.wad there are two TRAKINFO lumps. The lower one contains the hashes for original recordings that are binded to their places in the games and those recordings start with O_ and all of them are defined in the lower TRAKINFO. That does include the songs that play in the multitude of levels, like E1M7 (Demons On The Prey). The second, the one is higher on the list, contains only the variables to prefer a different track insted of O_ if the option to play remastered soundtrack is on. It's specifies the file to be replaced, like O_E1M1, with a brand new one - H_E1M1. It will only work if O_ file has a hash that is specifically tied to be played on levels, pretty much like it was in Chocolate Doom HD soundtrack installation. Due to that TNT has now the option to have all HD tracks, but that option is rendered useless for other expansions that simply strive on reusing music from Doom 1 and 2. The coding there is straightforward and you won't get lost.

Thanks to Seamus for figuring that out. It turns out he used this very option to change some tracks that were missplaced on the release, but fixed later.

Each wad containing a game or a mission pack has maps and has UMAPINFO lump as well.
That lump is works exactly like it works in any other port, with some quirks in KEX DOOM.
1. If you have added at least one "music = "lump"" line in the map configuration the rest of the music in the levelpack suddenly will change to play .mus files from the wad with OPL2 synth. So if this option is chosen the rest of the levels should also have the music option set in.
2. As it can only choose one specific tracks, that is the reason every addon except for TNT had to be split in two, since it overrides hashes reading, remaster vs original option and will only read the file directed in the UMAPINFO.
3. It will read files both from the current wad and extras.wad. For that reasons music for maps is stored inside extras.wad and Title Screen, Text Screen and Intermission alongside change UMAPINFO is the only thing there for installation fo chosen pack. The rest is always installed in extras.wad.
4. in the same section as music was added, to the author command line I added a track name and an author for community midi packs. It doesn't not show in the Intermission screen, but it is shown in the console on the startup of the level simultaneously with map author, which is cute.