as i understand it, in 5.1 renderings of films, the voices remain in the centre channel
and the music in the front stereo pair (with ambience and specials in surrounds) hence,
for this material, they are already separated (if I'm right)
That's correct. It is a common practise that the dialogue is placed only
into the
center front channel in discrete multichannel, such as Dolby Digital 5.1
and DTS 5.1.
You may find exceptions especially in comedies, when some short parts of
the dialogue
have been placed in some other direction, or panned between two channels.