One thing that jumped out at me from this video:
http://www.youtube.com/watch?v=Lg7DV1ji30Y#t=20s The four basically-identical "bang" lines are charted as alternating notes. Could just be how they handle talkies.
Edit: actually, watch through this trailer again:
http://www.youtube.com/watch?v=X85ycsb7Nhk I see no instances where vocals doesn't follow a consistent left right left right pattern. I believe it's based entirely on rhythm, at least at the point when they were making these videos. The algorithm may still be in progress.