BILL MITCHELL escaped from darkest New Jersey years ago but now he's back. Meanwhile, he became vice-president of an Italian jewelry importing firm, did fund raising for the New York City Ballet, and then computer systems for a very large and boring international corporation. Among his other peculiarities he is afflicted with a genetic abnormality that prompts him to remember musical pitches long after they have ceased to be sounded. This trait is of no earthly use except to an a cappella singer.