David Henry - International Language Center, Autonomous University of Guadalajara, San Antonio TX USA
My first encounter with English language sound peculiarities came in my youth when my family lived in the mountains of Maryland and cousins came to visit from Pennsylvania. I remember that for them the vowel sound in "roof" was that in "book" rather than that of "boot." Much later in life, thinking about the African-American dialect, I discovered the work of William Labov at the University of Pennsylvania who has created a map, "The Atlas of North American English: Phonetics, phonology and sound change." It was much too complex for my modest needs, but may be of interest to some on this list. ( http://www.mouton-online.com/ )
Talking with a speech language pathologist in Pittsburgh, I was surprised when she asked me which African American English dialect in Pittsburgh I was interested in. There were, she told me, three. Pittsburgh's African American population has immigrated from three different parts of the country.
So much for rules! We can analyze distinct populations, but we always come up against environment, both in the sense of the area of the country we're talking about and with reference to the neighborhood vowel and consonant sounds find themselves in in different words. I hadn't even thought about the effect of /s/ in words like "listen" which Dr. Bland draws our attention to.
While it is true that education has some effect on pronunciation, it seems to me that educated people are different in that they have the luxury of being able to code-switch: they are able to use both what they imagine "proper" English pronunciation to be and the down-and-dirty pronunciation most of us use in rapid and informal speech--just as many African-Americans can switch from African American English to Standard American English (whatever on earth that is).
While I suggest that students listen to National Public Radio to help with their various needs, I'm aware that they will hear brilliant astrophysicists, celebrated cardiologists, distinguished professors of this or that including Nobel Prize winners say something like: 'This book represents years of research by Professor Jones and I' and my favorite: 'This is Public Radio In'ernational.' So, educated in what?
Comments by listers demolish the myth that there is a firm set of rules for pronunciation or any firm basis for being "schoolmarmish." Bottom line is preparing students to listen and speak at two levels as each of us understands those two levels so that students won't be confused when they hear Elvis Presley sing: "I wanchyu, I needzyu." Students pay my school, the In'ernashionul Laengwidge Cen'er, to help them cope with life as strangers in a strange language, and to do that in a limited amount of time. Disappearing /t/, reductions including schwa, assimilation, and vowel sound differences are facts of life in North American English. All who have responded, while giving different weight to "high" and "low" forms, agree that students need to be aware of pronunciation possibilities.