Full Metal Jacket (1987)

The mono track on Warner's UHD disc is either a downmix of the 5.1 remix or a new mono mix prepared from the files that ultimately created the 5.1 mix. The easiest-to-spot difference is Paint It Black during the end credits, which was always slightly sped and pitched up in the original mono mix but plays at the correct speed in the 5.1 and new mono mix.

Of course, if you compare the original and new mono tracks more closely and align their waveforms, you'll see that there are subtly different sound effects between them, usually in scenes in the second half of the film with more vehicles and gunshots. 

I don't usually compare the waveforms or spectrals of different mixes, but I've done this below to give you an idea of how much less high-end exists in the new mix. It's a lot 'thicker' than the original mono (bassier, plenty of noise reduction) and even the 5.1. Someone at Warner clearly put in some effort to make it sound different enough to be disguised as the original mono mix. 

There's some clipping in the new mono mix that's also (incidentally) visible in the centre channel of the 5.1 mix.

As noted previously, the Japanese LaserDisc sounds worse than the 1999 DVD.

Full Metal Jacket (1987)

The mono track on the 1999 Warner DVD is outstanding.

It's much, much less muffled and murky than the mono on the 1989 Japanese LaserDisc. The difference is massive.

While segments still do sound better in the remix (it is, after all, a discrete remix done ~15 years later), these moments are pretty infrequent. The DVD mono doesn't exhibit any traces of hiss reduction or any other attempt to shape the intended sound, but there are many moments in the remix where this has been done. Dialogue in the mono is usually clearer and more prominent in the mix. The remix also has a lot of additional reverb added to the dialogue in indoor scenes.

But there's also sometimes a fair bit of compression on the dialogue in the mono. Still, I think the mix is excellent and better overall than the 5.1.