Well, of course, Madonna speaks well: her accent is the one God intended. (She and I grew up in the same town and went to the same grammar school several years apart and she speaks as I do.) Or, as Wendell has noted, that was the way she originally spoke.
More seriously, Madonna did grow up in a region (suburban Detroit) that shares the general dialect as the “Chicago” style of broadcasting mentioned above (as Midwestern English).
I would not go so far as to claim that that dialect has the same authority as Received English, but it is true that there was a concerted effort in the middle of the 20th century to get radio and television reporters to speak in that manner, and the practice carried over, to a limited extent, to acting. As late as WWII, there were regional accents in the U.S. that were nearly mutually unintelligible and it was felt (correctly or incorrectly) that the “Chicago” pronunciation was the one that was most easily understood by everyone. It has none of the “twang” associated with the two central U.S. dialects and does not contain the drawl of the deep South.
On the other hand, when encountering people from other parts of the U.S., I have often been twitted about our “strange” vocabulary and pronunciation of various words and I doubt that many people outside the Buffalo-Cleveland-Detroit-Chicago corridor actually believe that our accent is “proper,” the way that someone in Britain might acknowledge (however gudgingly) that the Received English is “proper.”
In fact, in recent years the whole “Chicago” approach to broadcasting has begun to lose strength, and you can hear many more regionalisms in both the news departments and in the movies than you might have a few years ago. There is still a certain pressure to avoid the more extreme accents, but there is no longer an effort to make everyone sound as though they were raised in Highland Park, IL or Grosse Pointe, MI.
For example, Dan Rather’s speech, when broadcasting, is much closer to “Chicago” than to his native Texan.
At the same time, the high mobility of U.S. citizens, even more than the force of movies, radio, and TV, has tended to moderate against any strong accents. It is hard to develop and keep a dialect when it is constantly subjected to immigrants bringing in their own pronunciations and vocabulary (while one’s own citizens are moving away to export your vocabulary and pronunciation).
“High class” might actually appear outside the realm of “proper.” High class accents would tend to be those of the Old Money families from Boston, Phildelphia, Savannah, and Charleston. (Among the wealthiest and oldest social groups, the accent in those cities is remarkably similar (they tended to intermarry and keep their separate dialect alive and somewhat distinct from the “common” accents surrounding them.) Such accents might even sound affected to some U.S. residents.)