There’s been a misunderstanding. The extended audio I was referring to was the humming still heard even after the fade to black on the DTS 7.1 mix.
In regards to the chorus, the only conclusion I can draw from the volume differences in the mono to stereo mix is that, again, when Disney prepared the downmix for wider distribution in 1941, the opportunity to play with the original elements to prioritize different elements of the score, once again presented itself, and they did just that. What’s heard in the stereo mix is essentially what was heard on opening night-- the mixing on the mono track was prepared after the fact, and its mixing is completely unique from the final stereo. Therefore, not originally what was heard on November 13th, 1940. Ave Maria isn’t the only segment in mono with striking mixing differences from its stereo counterpart. In The Sorcerer’s Apprentice, a cymbal clash is placed right over Mickey’s second dive into the water after he wakes up from his dream. In A Night on Bald Mountain, an almost muted but sharp, sinister brassy note can be heard just as Chernabog begins to play with hellfire. Dance of the Sugar Plum Fairies from Nutcracker? Just try lining up the mono mix to the original stereo track. Two completely different takes. Those are the few that come to mind. There’s plenty of other mixing minutiae that point to the mono mix being a total reworking of the original stems and not just a simple fold-down of the final theatrical stereo track, and I’ll try to point them out in some way in the (hopefully near) future, but trust me, they’re all over the place. The finale’s chorus mixing is only one.