lol., this is roughly the 4th time I've answered this ;) So here I go again!...
Well, every accent changes over time.
One theory is that it didn't change, but rather it is like a "snapshot" of an Elizabethan English accent. America was first settled by English speakers in the 16th century. The accent in America "froze" and the accent in England continued to change, (as well as the American accent). Some Shakespearean experts (and not just American ones) say that the language of Shakespeare works much better in an American accent, and this some proof to that theory.
So - the English, a good 400 years ago, sounded just like Americans (the typical American accent) do today. In the US, there is such a thing as a "Shakespearean" and "Elizabethan" accents -- which is theorized to believe the closest accent to what Shakespeare and Queen Elizabeth sounded like.
If you really listen to Southern Belle accents, you can really hear a mix of a Scottish and Irish (which is the base of the way all Southern accents were formed).
I live really close to Pittsburgh, PA -- and just about everyone here has a huge Irish twang to their accent. You can also hear a mix of Canadian as well. It's a really weird accent. lol.
Hope this helps! :)