What's Standard English? According to The American Heritage Dictionary it's the speech of educated speakers. So where do we find the model? Chicago? Miami? New York? L.A.?
A region without an accent? Fuhgeddaboudit.
Acting students learn to speak "unaccented" American, an idealized form of speech.
Elementary students in Pittsburgh learn poetry and to enunciate more precisely.
L.A. students play a version of the TV game where challengers translate phrases into SAE.
People who invoke the term Standard English rarely make clear what they have in mind by it, and tend to slur over the inconvenient ambiguities that are inherent in the term.
Where does this leave us? American Heritage suggests there's no single, universally accepted standard for how to speak or write American English. Even so, school systems, professional communicators and businesses all have standards and, not surprisingly, the rules (at least for grammar) do not vary dramatically from place to place.
What's more elusive is finding an accent that sets the standard. The variety of English spoken in the nation's Midland areas is often pointed to as sounding most neutral or "mainstream." It's frequently identified as the speech of broadcasters. But as linguist Matthew Gordon explains, it too is not unaccented English. For a variety of reasons, over time, the Midland variety may lose its status as the vox media.
The "unaccented" variety that is sometimes called Standard American or Standard Speech is one taught by accent coaches. This form is actually an idealized dialect - meaning, it's not really spoken anywhere, but instead is acquired through professional training. Actors and professional communicators (including some from the Midlands!) often take classes in "accent reduction" to lose any regional or social sounds in their speech. It takes a lot of work.
"Standard Speech" is spoken nowhere in America, as such. It is based on RP (British Received Pronunciation) which was adopted with American alterations in the early 20th century by linguist William Tilly. These alterations, this authentic "American" sound was loosely based on the speech of North Eastern population of the US. It was spoken by the cultured, well educated, well traveled people of the time. Listen to old movies to hear it.
According to Daniel Jones, An English Pronouncing Dictionary, "There are innumerable other ways of pronouncing English in existence, and I do not claim that RP is intrinsically "better" or more "beautiful" than any other form of pronunciation."
Rosina Lippi-Green. English with an Accent. New York: Routledge, 1997.