Isn’t there a lot more with original soundtracks that don’t include any 5.1 mix at all? Just going from my BD shelf, the first Vacation movies and Driver. I think many obscure films only contain the original stereo/mono mix.
the Vacation movies were actually never remixed, the first two are in mono and the third in stereo on their blu-ray releases, the fourth one is in 5.1 but my guess is that it was always in 5.1
However the first movie has a song changed in it but it’s been an issue since the second VHS