Contrary to the intro, Seems to me that I recall most old games using MOD and other tracker formats instead of MIDI.... but the point is that it was sequenced samples, I suppose.
As far as I can remember, using MOD music was pretty rare for MS-DOS-era PC games. The only exceptions I can think of were either Epic MegaGames titles (Unreal, Jazz Jackrabbit, Epic Pinball) or games that originated in the demoscene (Zone 66).
I guess it depends on the era. Most of the computer games of the early nineties relied on MIDI, although their Amiga ports did use MOD of course, being the de-facto Amiga music format.