I may be completely wrong so correct me if I am. I dont really know what I'm talking about lol!
Basically, I was reading a few months ago about the creation of acapellas and they were made by having the original song and an instrumental at the same bitrate in phase with each other (possibly with one reversed or something similar - search google, theres loads of stuff on them) which left only the vocals remaining.
Therefore, on your dvd, could you get an original copy of the song at the same bitrate as the film, do the special tricks to it and it only leave the voices in the film as there is no removal of that?
This is just an idea I thought up and I really don't know if it will work, but someone will correct me I'm sure!