From what I can tell, the GOG version of
Quake does indeed include the soundtrack... if you run the original DOS software renderer. By default it seems to use a GLIDE port (...interesting choice) that doesn't play the music for whatever reason. That said, the music seems to be misaligned a bit - using the "CD PLAY
n" command in the console, I've noticed a second or two seem to get cut off of the start of most of the tracks - most glaringly, the level clear music, which is
very noticeable.
Although really, if you're playing
Quake these days, it should probably be with
Quakespasm, with
Quake Injector for more custom maps than you can shake an axe at. The old, official hardware renderers have proven to have a few graphical deficiencies over the years. Namely: some of the palette is supposed to be 100% bright all the time as per the software renderer, but isn't in the hardware; walls close to light sources are supposed to get "overbright", and do in the software renderer, but don't in the hardware... both of these are fixed by Quakespasm. Also Darkplaces, but Quakespasm has better support for newer custom maps than Darkplaces does, if you can believe that.