I was also thrilled to see Christopher Ryan as the Sontaran. His voice is perfect, and he gets that "pugnacious short guy" thing down perfectly.
As for the Auton gun effect, I know what you mean about that. I liked the old effect, it's very unusual sounding. But maybe that's why they didn't use it - since they weren't showing the actual laser beam hitting her, perhaps they were concerned that people wouldn't identify that weird "bonk-whoosh!" as the sound of a gun? (They could have had a practical red flash effect, which would have helped to sell that better.)
Or, maybe Moffat just doesn't like it.
As for the whirring mechanical noises, other than the sound of their gun-hands activating (a classic series effect) there's really no reason for them. But, again, perhaps they were concerned that "the average viewer" would need that extra bit of audio to clue them in that these were constructs of some kind? Though it's doubly silly that they only make little servo-noises after their identities are revealed. Did they have their legs on silent??