Jeez, glad I haven't considered getting rid of my DVD... I wonder how the vocal got swapped on "Frankenstein Place" if it was right on the DVD 5.1. It otherwise seemed like the vocal and music mix elements were the same on the Blu and DVD, right down to that new Time Warp ending. I can't imagine someone went back to the multi-track tapes and completely edited the music from scratch after all that detailed work on the last 5.1 mix. That last mix entailed painstaking comparison of the vocal takes on the session tapes versus the mono dialogue stem, mixing them together within songs when needed.
I could see using stems or digital audio sessions with multiple tracks... hmmm... maybe if they did work from an existing digital multitrack project, there could have been two vocals in there and the 7.1 team just picked the wrong one?
Here's another interesting thing to note about the music - if you pick the karaoke option and play songs individually, you can turn the vocals off. If you do this, what you hear is the music and effects stems of the mono mix. You'd think it would be easy to output a separate pass of each song in 7.1 and just eliminate the main vocals, so it's interesting that this was the route they chose.
EDIT: Now having gone to David Shetterly's review and discovering some links to a bit of "behind-the-scenes" info, we can get a better insight into the audio process on the "Rocky Horror" Blu-ray:
http://www.rockyhorror.com/participation/behindtheblu_audio.php
Might I suggest as a companion my own interview with James Young at Chace who supervised the 5.1 mix for the DVD release? 
http://minerwerks.com/rockyremastered.html
Edited by Derek Miner - 10/21/10 at 11:27pm