As a life long Mid-Westerner, I can tell you that just within a few states, it varies so widely, it's hard to count.
Chicago, Upper Wisconsin, Northern Iowa/Minnesota/North Dakota, St. Louis, and what I'll call Common Southern Iowan are all very different. In fact,
on linguist sites, different areas of Chicago are separated.
But to the poster who said Californian is the "American accent", I would say that the SoCal accent (i.e. Jesse James) is totally foreign to what I
would expect as non-regional. They say "so" like "sew". Weird.