What would happen if some of the greatest voices to come out of Chicago joined forces on a single stage? You’d hear more than just jazz and rap, gospel and blues, classical and folk. You’d encounter the sound of the city itself, for Chicago has nurtured and shaped all those idioms, and then some. Without the blues innovations of Muddy Waters and Howlin’ Wolf, the gospel thunder of Mahalia Jackson, the folk incantations of Steve Goodman, the jazz seductions of Nat King Cole and the contributions of uncounted other Chicagoans, American music would be much the poorer. Anyone who doubts that proposition probably was not at the Civic Opera House…