I haven't looked at this one but there is a metadata format for WAV that many games with WAV soundtracks on GOG use, however not everything understands it. In particular, the flac encoder doesn't and the developer suggests ffmpeg, but I don't want to spend a week to figure out how to duplicate the settings I use so another option is to use kid3-cli to copy the tags. Early GOG and maybe the old physical distribution by CD Projekt did that when they could as goodies. A few even now provide audio files as separate files in the game distribution (most Falcom and Arcen games at least) but usually without metadata and some editing/selection that goes into official soundtracks. For that matter I've seen a few recent indie games that include the soundtrack as a goodie (I can't think of an example at the moment but I'm sure I've purchased at least one or two games because of that).

I guess there is no clear standard for it, so you can't easily use it.

Yeah, that'd explain a bit. System Shock's original music really depended on a lot of things to even sound like music, but at least it wasn't quiet.

mpv and vlc understand it just fine. But converting it to flac and copying the tags saves disk space and makes the tags fully standard. It would be great if they came in FLAC since you get the exact same thing in half the size (and I have a slow connection).

Early GOG didn't really even carry many "somewhat recent" games, let alone brand-new (apart from CDPR's, for obvious reasons). The person I replied to raised this issue in this thread related to this brand-new game -- because, remake though it may be, it's still a from-the-ground-up remake, with a completely new soundtrack (minimal and underwhelming though it many seem to find it).

But speaking of those already-old-at-the-time games that they released with bundled soundtracks and other goodies 12+ years ago: the only reason they went to so much trouble getting all those extras together and bundling them in with the respective games was because they wanted to entice people who weren't used to buying purely digital goods to come shop here (because it was by no means clear to anyone that a DD shop that only sold old games -- without DRM -- would work). That was their hook (well, one of a few :P ). But once publishers, devs and GOG themselves realized that most people would still buy stuff even without extensive goodies (and sometimes without even basic, necessary "extras", like manuals for most games from before the mid-'00s), they trailed off in providing those even for the oldie re-releases. Much like how Steam and their major partners cooled it with the giveaways and frequent super-deep discounts (so I've heard), after they achieved market hegemony.
Don't get me wrong, I can understand the wistful remembrance of marketing strategies gone by, but one needs to recognize that that's what they were. :)