If you want my theory, its that the slew of rewrites between 1980 (when this episode of WWOD was made) and 1984 probably forced Disney to toss Mills' recordings in favor of Trillian.
There are at least three versions of Trick or Treat. Not as many as say...Three Little Pigs, but still it was a pretty common reissue (burlap or otherwise).
I suppose the gallery (I forgot that was on there, just shows how unmemorable the old 30th Anniversary Edition was) is useful since there will likely never be any coffee table books or behind the scenes material released on The Black Cauldron (because again, its "that movie we don't talk...
If there's anything good that can be said about The Black Cauldron, its that the film marks the defacto end of the Disney Dark Age. It had to be, the film was rock bottom for the studio. It was only up, up, up from here until the studio began another slow downward spiral with Pocahontas that...
Sleeping Beauty stated out as a Cinemascope production and composed for that with magstripping only. They changed the photography process to Technirama late in the game to take advantage of its ability to be converted to 70mm (this was pre-blowups mind you) but the art assets were left at 2.55...
The difference in this case is Black Cauldron had next to no special features on it except for a trailer and a deleted scene so its not like you're missing much. Its that black sheep status again.