I've heard the stormtroopers lines are pre-recorded and not a voice changer. There seem to be a lot though and they can usually follow conversations. How do they select which lines to play?
That's a really good question, I heard that their hand movements(?) had something to do with line they say but, I'm not sure how they remember all of them and are able to pick out the "right" one so quickly.
Hand movements. Watch hand gestures of Storm Troopers and Darth Vader when they talk. This was explained to me by a friend of a Storm Trooper a few years ago.