This is an MP3 conversion (lower-quality, of course) of a 96/24 version of the original mix, with expanded dynamics. The original mix was mostly 48/16 and it took a month of tweaking and tuning.
This is Descent.mid played with the Silverspring Soundfont 2 bank. The soft synth is BASSMIDI Windows driver. The BASSMIDI library developed by Ian Luck of Un4seen Developments, this is a Windows driver port that works as a system MIDI driver under Windows XP/Vista/7, and probably 2000.