I now own both the BR Director's cut and International LDs, so I'll sit down soon to compare. I primarily got them for completist's sake and for the fact that they feature the original Dolby matrixed surround mixes instead of redone 5.1 tracks.
2001 has two original versions: 70mm six-track Cinerama mix and the 35mm version which I saw, and that may have had some type of extra audio...because it certainly sounded like it! Criterion used the original master and working with Kubrick and his editor (while making Full Metal Jacket) put it straight into Dolby ProLogic for surround, and used the 35mm master (2.21:1 special print). Not sure what MGM used for their versions but it seems similar if not a bit cropped. The new mix was done for DVD and re-purposed into a 5.1 mix that tried to replicate the original 70mm intent. This has either been re-utilized or spruced up a bit for the Blu-ray. Good mix, but I might prefer the Criterion PCM a bit as it sounds closer to 1968.
North by Northwest, I can easily answer. The Criterion LD is the only edition to feature the original mono track, and PCM only on the CLV release (Which I have.) It was never for certain until recently if the film ever had a stereo or matrixed roadshow release in 1959, this issue came up on the Steve Hoffman forums, and I wound up finally getting an answer: Mono only.
http://www.stevehoffman.tv/forums/showthread.php?t=280234&highlight=north+northwest+mono&page=2
The 5.1 mix is quite good for a 50's mono film, but it's really not that necessary and to me the mono sounds much more balanced and natural. The score was recorded in stereo, but it sounds fine in mono.
I don't own the Apocalypse Blu-ray, nor is my receiver HD capable but from reports it is supposed to pretty much replicate what the film originally sounded like. However, I adore the 1991 widescreen LD PCM as Walter Murch essentially just dumped the '79 master audio straight to Dolby ProLogic Surround, giving an almost exact replica of the theatrical audio. The later 5.1 remixed LD sounds lowered, as they were trying to promote the new discrete ac3 mix of the PCM, and thus weakened the PCM original. Neither sounds as good. BTW the tiger on this LD audio will scare the pee out of you.