Anyway, Murch put a lot of his thoughts about both editing and sound into a book called 'In the Blink of an Eye', which is still read by film students today. One passage that always stuck with me is an informal rule that Murch gave himself after he discovered something odd about the interplay between sound and audience immersion. After a lot of experimentaiton, Murch found that in any 'moment' of a film (say 3 - 7 seconds), an audience can only process 2-3 layers of sound playing at once.
For example, Michael Corleone goes to meet Moe Greene and they're walking through the lobby. We hear footsteps, the elevator ding, and the atmos of the hotel. If Murch had added the sound of he luggage of the bell-boys or some guests arguing in the rooms, it would be too overwhelming for us and the verisimilitude of the film would become compromised.
I guess I'm mentioning Murch because this informal sound rule was always independent of speech from the actors, which he doesn't treat as a layer of sound. To me, this may be a very practical example of what these researchers seem to be finding: that there is indeed some parallel processing going on with speech and sound in our auditory cortex.