Please add wallpapers, arts, and a manual if possible. The soundtrack would be nice too, either as a goodie or separate DLC

Seconding this. The soundtrack is (I think) done by the late Steve Henifin of Silicon Knights - he did the OST for Blood Omen: Legacy of Kain, Eternal Darkness and many more. Maybe GoG can see if the rights holders or former developers have any promo art or anything left over (you can see some of it adverts and previews of the day).

Yes. I never heard of this game before it appeared on GOG and when I checked out gameplay on YT, I was surprised at how good the soundtrack and artstyle was.

I don't think the soundtrack had a separate release, but the game was originally released in Yellow Book format. So, if you go to your installation folder (assuming you bought the GOG release), you should see a "music" folder. All the tracks are there in OGG format.
I know it's not as good as having the tracks ripped in FLAC, but the game is pretty obscure. I couldn't find the soundtrack on neither Amazon Music or Spotify.
And if anyone here owns the original release on CD and feels like ripping the music for the rest of us, EAC is probably the best software to do so.

Please add the map, it helps not getting lost.