After some more testing, I found some weird things going on with the OPL2 KSL ROM that was taken from Dosbox or something like that I think (I don't remember which) together with the other volume envelope logic.

Somehow, when I move the KSL lookup table outside said function (which is displaced by 8 apparently, based on the ROMs), the sound for OPL2/OPL3 goes haywire for some OPL3 testing songs I used? Can't seem to figure out why that happens.

Managed to get NT 4.0 workstation booting properly again during setup (phase 2, so after the formatting and copying the OS files over for the graphical stage of the boot). 😁