General observations: the Nelson LD has the loudest dialogue and sound effects relative to its music, while the Criterion LD and the 5.1 have much louder music. The Criterion LD has the poorest fidelity -- its dialogue is mixed like a giant wall, with a bit of its high-end shorn off. The 5.1 is slightly better, but not by much. I really dislike how the dialogue and sound effects are muffled and rolled off on these two. The Nelson LD is crystal clear, but almost anaemic in how unmanipulated it is -- no exaggerated bass, no compression to the dialogue/effects/music.
Kudos to the fellow who pointed this out: at 00:23:57 (Criterion blu-ray timecode), Wesley's sword is now punctuated with a musical sting as it enters the ground. This effect is absent in the stereo mixes, which just have the diegetic sound of the sword lodging itself. Other minor variations may exist, but this is the only one that's noticeable to me without A/Bing obsessively. (I'd say the 5.1 is very faithful overall to the earlier mixes' sound content.)
The Nelson LD track is really atypical of how most film soundtracks are mixed nowadays, which usually have everything at more or less the same volume. Is the Nelson the theatrical mix? No idea, but it's my easily my favourite.
(It would, however, be logical for Reiner or someone in the film's production to try and bury an original mix they were unsatisfied with, replacing it with ones that are less 'flat'.)
