Excellent thread and work on this! You strike me as an expert on these matters, so I wanted to ask you what is meant to be the very original theatrical mix? I’m from the UK and I’ve got the UHD and Blu-ray, the former containing the Atmos track and English Dolby Digital 5.1, the latter a 5.1 DTS-HD mix. The DTS-HD mix, to my ears, sounds incredible, and its frustrating that - like many of the other Pixar films on UHD in the UK - it isn’t included on the 4K. The thing is, I’ve been informed that the Dolby Digital track found on the UHD is supposedly the original theatrical audio track. Maybe I don’t understand, or maybe Disney chose to neuter the audio, but I watched through the film with the DD track and it didn’t sound as robust as the 5.1 or how I would presume such a track as that would sound. So I guess my question is: what is meant to be TS1’s original audio? Is it a 5.1 DD or something else entirely? And if the DD is the original track, is it me or the disc that is resulting in a lesser sound? I have this same issue with a handful of the other Pixar UHD audios, like Monsters Inc., which also has a robust 5.1 on the Blu-ray and not the UHD which contains a 5.1 DD that is likewise meant to be the original theatrical audio. Thanks for any information you can provide!

As far as I’ve been able to make sense of it, the general evolution of its English 5.1 mixes (the sound effects only one notwithstanding) has been something like this:
Dolby Digital 5.1 - the Original Theatrical, continued into the 2000 DVD release
Dolby Digital 5.1 EX (6.1 remix from 2005, 10th Anniversary Edition DVD; now on the UHD)
DTS-HD MA 5.1 ES (2011 Blu-ray / 3D Blu-ray)
DTS-HD MA (2011 Blu-ray)
Dolby Atmos (2019 4K)