OK it look like gog have renamed winmm.dll to _winmm.dll
So if you want to re-enable playing back the music from .ogg files it would seem that you need to do the following.
1. Mount the CDIMAGE game.gog with daemon tools or cd emulation software of choice.
2. rip tracks2 - track11 with CDEx (or other ripping software of choice) as ogg the "music" folder inside your gog quake folder as Track02.ogg - Track11.ogg
3. rename _winmm.dll to winmm.dll
4. run qlquake as normal from the icon and enjoy the music.
A little further tweaking will be and ripping the music from the expansion images will be required to get them playing the correct music but that should not be to hard.
The same ogg music files also work with quakespasm and it will automatically play the right music if you follow the instructions in Quakespasm-Music.txt
Its good to finally have a legal way to purchase Quake with the expansions and get all the music :-)
If people are still stuck I am sure if would not be that hard to create a .bat file that automatically rips the music from the iso images and saves it in the right place, now we know the required steps to make things work. But I don't have time to look at that at the moment.
I'm an eeeejit. I was going to post that I essentially already tried this (albeit using the Outlaws version of the dll; I didn't realise it was included but renamed for Quake!) and that it didn't work x 2, and then... I noticed my files were renamed wrongly. 002.ogg instead of 02, for example. Renamed and works.
Still.. if the music goes into the, erm... 'MUSIC' folder instead of id1, rogue, etc, then how are we meant to get the correct music working for each mission pack? After all, there can be only one 02.ogg, for example, thus whichever one used will play regardless of the game or pack. I'm guessing that's why even the original installer had issues with incorrect music for the packs; GOG themselves didn't/couldn't fix it.
Using the named folders as you describe above works with quakespasm as I said.
I think to get the the old glquake to use the right music would would need to do the following:
1. create 3 different music folders one for the base game and one for each of the expansions, the rip the music into the different folders
2. either rename the music folder you want each time to music manually before running quake or create a bat file do the renames and run quake after doing the renaming the correct music folder so it plays the correct music.
Personally I think I am just going to use quakesasm as it seems like the easiest way to get the correct music and hardware 3D.