When two groups of people who initially have the same language become isolated, each group's language changes in different ways. That's essentially what happened when some English speakers migrated to America, and some stayed in Britain. Neither modern accent sounds like it did 400 years ago.

(Also, you'll notice people in the southern US have different accents from the north? That's due to the origin of the settlers; settlers came from different places in Britain that had different accents, so they brought their local accents with them, but each group that travelled together was generally from the same area.)

sources I remember reading this stuff from: Language Myths; Contemporary Linguistics; Language in the USA

You might have better luck asking this question over at /r/asklinguistics. Curiously, a quick search over there did not return with any relevant topics.

Just predicting some of the responses you might get, no modern American or English accents are exactly the same as they were in the 17th century. Some characteristics were preserved and some altered differently to any given region over the years. You might have better luck asking what this characteristics were and what caused the shift if you are looking for concrete examples rather than having people repeat what I just said in a way more eloquent way.

Another thing to remember is that there are loads of different accents both in America and Britain and any change that happened to them was due to multiple factors including geography, contact with other languages, the society at the time and even fashion. So to analyse any given modern American accent you'd have to consider what dialect of English the current American accent descends from and what influences and processes might have caused them to diverge individually over the years. So focusing on a specific accent would help as well.

Hope that helped.

From the /r/linguistics FAQ:

http://www.reddit.com/r/linguistics/comments/1lhiyl/how_and_when_did_americans_lose_their_britishuk/

Follow up question: has anyone reconstructed 17-18th century english? What did the common English sound like at the time at say, mount vernon?

There's been some related questions asked here in the past, perhaps you can find an answer in one of these:

• Would Americans at the time of the revolution have spoken with a distinct non-British accent?
• When did the current dialect of American English diverge from traditional British English?
• When did Americans lose their Brittish accent?
• Differences in American and British English accents
• At what point in time did Americans develop an American accent (or accents, for say the north and south)?

The short answer, however, is that there's not an "original" British accent in the world today. After colonization of the Americas, the British and American accents (and there's several of each) diverged. So each group of accents has some areas where they've preserved features of the older, common accent, and some areas where they've changed.