I understand that Standard American English is rather recent, as there are many "American accents," but every time I listen to old US radio broadcasts, I just notice that these radio broadcasters are clearly American but still sound quite different.
I don't mean to discourage any response here, but you may have better luck with this question over at /r/linguistics. If you aren't able to get a satisfactory answer (or even if you do get one and still want more!), I might try asking again over there.