for the most part americans talk like the people surrounding them growing up. if they intentionally want to change their speech, they may choose someone on tv to sound like, but generally speaking that's rare. southerners sound like they're from the south, no matter how much tv they watch, bostonians sound like they're from boston, same for new yorkers, minnesotans, cajuns, etc
edit: if you want to hear a fantastic minnesota accent, check out that movie Fargo