okay so, i finally found the DOS FPS game i'd played the demo of as a kid!!! i could remember it being in 3D, with configurable sound card options, a kind of sci-fi รฆsthetic, some outdoor sections with brown being the predominant colour, but also prominant use of electric blue...
based on those details i knew it had to be from the mid-to-late 90's. sure enough, it's Domination (1998), apparently also known Rex Blade: The Battle Begins.

playing it now, i understand why I didn't keep playing: it sucks.
the reason i was reminded of it and wanted to find it again today is that i remembered the sound card options, and wondered if it had General MIDI i could try out with my new (old) MIDI module. sure enough, it has both FM synthesis and MIDI, and both OSTs suck. lmao.
BUT.
update! i've recorded an extensive video of the thing that the post above is about: https://www.youtube.com/watch?v=cC8oECLUXzs