I'm with Alan on this one.

In my experience, it is an American thing (mostly). Watch anything from the good ole U S of A, and you can't understand a word they're talking about. A combination of bad diction and loud background music, plus rubbish sound quality.

In contrast, most UK drama is much more subtle and you can hear the words.

I'm sure there are exceptions, but it largely holds true.

Oh, and whilst Bernard is right in what he says, I find it even worse at the cinema, which is the medium which the films are designed for.