So I've noticed that when I play a lot of my old MID music files (for Microsoft Ants, Age of Empires, and Age of Empires II) on new computers, they sound A LOT different from what they sounded like on an old 1997 computer of mine. In fact, I preferred how they sounded like on my very old 1997 computer. Has anyone else noticed this? Does anyone else know how I could try to recreate those sounds as they sounded like on a 1997 computer? (since that computer is long gone now).

Yeah I have had that problem switching midis to new (After Window 95) computers. I think it relates to the tracks "forgetting" what instrument sound they were supposed to make and substituting a new one. Or just different codexes assigning different instrument sounds.
What I've found has helped for me, though it's tedious, is to use a MIDI editor to manually "fix" the song. I did this mostly with MIDIs I myself had created.
I forget what program I used, but just google "freeware midi editor".