I've never noticed any glaring dialogue problems (or lack of dialogue) in ESB, though it's been a while since I've watched it. I do agree with Ronster's point that the voice that precedes C3P0 getting blasted sounds nothing like a Stormtrooper, but I just assumed they did that so that we would have no idea what person or creature shot him. When Chewie fixes him and he starts babbling about Stormtroopers, we're all like, "Sure would have been nice if you could have shared that yesterday!"

I'm not suggesting that the voice they used was the right way to handle it, but I always just accepted that scenario from a storytelling perspective.

EDIT: Thinking about it some more, you could replace the line "Who are you?" with some kind of short growl (still sounding like it could be human). The ST grunts off-screen, and C3PO's reaction tells the rest. We can say his helmet's off, which is why it wouldn't sound processed through a speaker like we're used to hearing. It's a little detail, but that's one way in which it could be addressed if it really bothers an editor.