Northern Cities
Fargo - Chit Chat

Having experiences traveling around the country, many people find Minnesotan accents to be a novelty. In this clip from the movie "Fargo", two Minnesotan men are discussing a recent homicide in the town. The example does a great job of portraying the conversational mannerisms and (overwhelmingly polite, questioning, not all that descriptive, and full of town references) that one finds in Minnesota, North Dakota, and Wisconsin.
Speech Accent Archive: Chicago
Play audioA speaker from Chicago (English 82) from George Mason University's Speech Accent Archive. a female aged 50. She displays stages 1 and 2, and variably stages 3 and 4, of the Northern Cities Shift.
Star Trek: Yoopers

A YouTube video that spooks the "Yooper" accent (Upper Peninsula, Michigan)
NPR: American Accent Undergoing Great Vowel Shift
An NPR interview from 2006 with William Labov about the Northern Cities Shift.
Northern Cities Vowel Shift: How Americans in the Great Lakes Are Revolutionizing English
A 2012 Slate article on the Northern Cities Shift and the diversification of American regional dialects.