During sound editing, his dialogue track will be split onto different tracks with the helmet on and helmet off. Most likely, the on-set dialogue that would have been recorded with his helmet on would have been from a small lavalier inside the helmet, and usually would be replaced by ADR. The audio in the rover would have been boomed (I would guess) to give it more of an open feel; although it is possible that they used a lav on set for that as well. But in post, we would separate out the helmet stuff from the rover stuff onto two different tracks. Then, using ProTools and various "plug-ins" to create different EQ and different amounts of reverb -- the sound mixer would create the different "feel" for the two sounds. In the mixing theatre, it would just be a matter of finding settings that implied a different sound texture -- yet would still be clear and audible for the audience. Opposed to using just the actual lav mic hidden inside the helmet, which would probably have way too much breath noises and actual helmet acoustics.
