The acting wasn't that great, although it wasn't terrible. Just average. Could've been better, could've been worse.
I didn't notice any accent problem. Probably for the same reason Sam Worthington apparently couldn't get a consistent one.
As for the audio, well, sometimes it's best if you don't even have to think about it. If it seems like it's just there, without taking your attention where it shouldn't, then it's good. As soon as you start to notice the audio as separate from the visual (e.g. if you are thinking that the sounds don't quite sound like you would expect them to), then you're losing quality. I didn't notice the audio at all, as separate from the movie. I never once even considered its presence as an individual element. So I'd say that any audio problems would probably have to be put down to the cinema in which you saw it.