Help me to create a list of GOG games with soundtracks included as goodies, or their music available on some installed folder in a format the music players recognize.
Games with soundtracks sold as DLC are probably off this list. Present those cases with details.

Something else for your consideration: Deluxe, Ultimate, Gold, Collector and else editions are ways to pack all the DLCs, plus soundtracks, plus art, plus wallpapers into a single pack. I think they can be found easily searching the word EDITION. Maybe there is no point to enlist those here. In case you have a different opinion, mention your reasoning to let the audience discuss about it.

Please consider that this listing belongs to everyone and, at the same time, to no one. Feel free to submit your suggestions.

Let's start with Gothic and Gothic 2.

Deponia. MP3. It includes the English and German sountracks separately. The titles and artist of the songs are missing while the rest of the metadata is fine.

Legend of Heroes series. A lot of the games use .ogg files you can play or replace with your own music choices.

But it is yours. You created it and volunteered to maintain the list. ;)

Fallout 1 and 2 used to have the OSTs available for download until it switched to the Bethesda versions. To get them, you can root through the files, but they're old ass file types you'd need to convert into another format.

Alternatively, if you have Fallout New Vegas downloaded, rooting through the files gets you MP3s of not only the New Vegas soundtrack and the licensed music, but 1 and 2's as well. And they're all there, minus the licensed opening tracks to those games.

SWAT 4 also has all/most of its music in .ogg format in the files, as does Unreal Tournament 2004 and the Quake remaster.

Vampire: The Masquerade - Redemption and Bloodlines have their music in MP3 format in the files too.

E.Y.E. Divine Cybermancy, SimCity 4, Morrowind, and Arcanum also have their soundtracks in their files.

As far as I can tell, all these games listed don't have official GOG releases you can download separately, and the files don't need to be converted to listen to them.

I appreciate it. However, I expect the people posting games and instructions where needed. If I volunteer to collect everything in a single list, it won't work. Sooner or later I'll start to fall behind. And with the effort there is also the possibility that I start to believe I am the one who dictates the rules. It is better a list that belongs to everyone and at the same time, to nobody.

Other games having their soundtrack as extra:

Sanitarium. MP3. The titles of the songs are missing while the rest of the metadata is fine.

Venetica - Gold Edition. MP3. The titles of the songs are fine while the rest of the metadata is missing.

Dex. FLAC and MP3.

This War of Mine. FLAC, MP3 and WAV.

Loria. MP3. The titles of the songs are fine while the rest of the metadata is missing.

Warhammer: Shadow of the Horned Rat. MP3. The titles of the songs are missing while the rest of the metadata is fine. The game also includes as extra 40 MIDI tracks.

Saint Kotar: The Yellow Mask. MP3. The titles of the songs are missing while the rest of the metadata is fine.

Undertale has the in-game soundtrack as .ogg files in a subdirectory of the game folder (albeit without metadata).

Worth noting that this is not the same as the official OST release. In particular, the game data includes some unused music not in the OST (kingdescription.ogg is probably the most interesting one), while the official OST contains at least one track not found in the game data ("The Song That Might Play When You Fight Sans", I believe it's called).

Painkiller has mp3 files in the Data/Music subfolder, and the soundtrack is also included among the goodies.

I could post a list of all my GOG soundtracks, then you only have to filter those which are DLCs :)
Because I really can't remember what I got with the game and what was a DLC.

The Ys games are typically like that as well. Ys VIII's files were originally scrambled but that was fixed, apparently. https://www.gog.com/forum/ys_viii_lacrimosa_of_dana/what_happened_to_the_music_in_the_bgm_folder

The same doesn't apply to every Falcom game universally, though: looking at what I have installed, I don't see any easy way to find Zwei's audio files, and Gurumin uses a different format.

I remember that, at least for one of the games, Ys Origin, the music filenames are numbers. Furthermore, I remember that number 13 is suspicious enough that the choice of which song has this particular number had to have been intentional.

Ohhh I can get on board with this thread. Enough with the shaming, more of the appraisal.

Games with extra OSTs free of charge

I'm not sure if some of those games aren't a Special Edition of sorts (unless otherwise noted), but I guess few are, I don't usually buy special editions. All display the soundtrack in my account.

Prince of Persia (2008) OST (Inon Zur)
(Not the best recording, still awesome)
Shadowrun: Dragonfall (John Everist)
Evoland 2 (Camille Schoell)
Unavowed (Thomas Regin)
Shardlight (Nathaniel Chambers)
The Inner World (Christian Barth)
Transistor (Darren Korb)
(Pyre and Bastion too, probably, but I have those OSTs on bandcamp)
Technobabylon (Nathan Allen Pinard)
Whispers of a Machine (Jacob Lincke)
Greak: Memories of Azur (Oscar Alcalá and María José Félix)
(this should be properly mixed to be much louder though)
Broken Sword: Director's Cut (Barrington Pheloung)
The Book of Unwritten Tales 2 (Benny Oschmann)
The Longest Journey (Bjørn Arve Lagim)
Dreamfall (Leon Willett)
Beyond Good & Evil (Christophe Héral)
Still one of my favs!
Edge of Eternity (Cédric Menendez and for a whopping 5 out of 74 tracks, Chrono Trigger Legend Yasonuri Mitsuda)

Honorary mention: Legend of Grimrock (it has the main theme as an extra, which more or less is the entire score)

Nine years ago, IAmSinistar and I made a thread about soundtracks included with and missing from GOG releases, which we maintained for quite some time with the help of a good number of fellow users, and which offers a sleuth of info

The thread can be found here.
Of course, since this was quite some time ago, all the releases post that time aren't included.

There are three noticeable issues:
(I) After GOG made some changes to the forum software, lots of links are now quite messed up, but they're still functional with some patience and persistence.
(II) Some of the links point to a different thread, which is now part of the archived General Discussion sub-forum. So in order for the links to work, the "general" part of the url must be replaced with "general_archive".
(III) Given that the thread is from quite some years back, I don't know if and how many of the external links still work today.

Perhaps some peeps will find something interesting.