As much as I love PP I doubt there was any consideration given on their inclusion to improve the acoustics.
There was.
Putting it bluntly, we could have built a 'noisy' stadium and had to locate it further out of town. We went back and forward on lots of aspects of the design in that period leading up to Christmas of 2004 and eventually, as deadlines pressed and differing technical/cost/environmental advice came from all directions, I remember asking for all the options to be summarised in plain-english for me and I took them home over Christmas. I do recall that Acoustics and the Stadium Roof was defo one of them.
A couple of days of peace-and-quiet, Morecambe and Wise, Queen's Christmas Message and Brussels Sprouts worked wonders with all decisions made by New Year's Eve. (most of them either wrong, right, inspired, ridiculous, brilliant or stupid depending on which forum thread you read)
Anyway, it's home now and it isn't serving us too bad.

J
PS. There is some suggestion that at PV the sound from the terraces may actually be being pushed through the amps around the localised speakers in the ground. It certainly is done in other venues. It works very well as a technique and certainly boosts the atmosphere. We're not sure, but someone I was with who knows about such things believes it was.