"MikMod" is a
module playback engine, of which there must be a
million different source
forks.
It used to be a popular sound engine for demo coders because it was one of the first freely available players with .xm support.
One version that was particular popular amongst wannabe democoders was the turbo pascal edition, MikPas. Unfortunately, since it needed to run in real mode with 640k ram it only supported the Gravis Ultrasound. Not that the GUS is a bad soundcard, far from it, but it's a shame all those old lamerproductions aren't as accessible anymore. (Yes, I know about GUSEmu).
.UNI is the internal mod format used by MikMod, but not all players (MikAmp for WinAmp comes to mind) supports loading this internal format, which is also a shame since you can rip quite a few good soundtracks from demos which unfortunately are in .UNI format.
The original versions were written for DOS and direct hardware access, and could be linked from asm, C and pascal. Later versions were ported to unix and java.
Later, better (as in more accurate playback - true-to-the-tracker playback, higher quality and better hardware support) such as the Inhouse Music System (based on the same codebase as CubicPlayer, which used to be closed source at the time - but now is available as OpenCubicPlayer and possibly has fallen victim of bitrot) were preferred for DOS Demos.