From the Director's notes by Nick Pegg.
The babble of voices inside Skagra's Sphere offered quite a complex challenge. Aside from the occasional 'spot effect' such as the Doctor's voice surfacing in the Sphere towards the end of part three, it didn't seem appropriate that we should be able to discern individual words. At the same time I certainly didn't want the cheap cop-out of a couple of bored actors muttering "Rhubarb, rhubarb".
So we went to some trouble to create a multi-layered effect, and recorded a great deal of material, ranging from ghostly whispers to ear-splitting shrieks and yells (not wanting to ruin the valuable larynxes of my cast, I provided the latter myself). Gareth Jenkins, our sound designer, then layered up all the different voices during post-production to create the effect of the babbling minds trapped inside the Sphere.
In addition to the 'general' Sphere voices, we recorded some specific ones too: I got Paul McGann to improvise a wildtrack of the Doctor's mind trapped in the Sphere (which you can hear in part three), and I also decided to record some material involving the Motorist and Professor Caldera.
Bearing in mind that the Sphere is supposed to contain every aspect of its victims' minds, I reasoned that we should hear not only howls of anguish, but also some idle, random streams of consciousness, so I asked Stuart Crossman and Barnaby Edwards to improvise the rambling thoughts of the Motorist and Caldera.
I should point out that these clips were entirely unrehearsed and unscripted - and as you can hear, the results were rather splendid. And yes, they were indeed used, however subtly, in the final version!