Seventy-five years ago Wednesday, two men, an ocean apart, stepped up to microphones. One man was a cello prodigy who had played for the queen of Spain. The other was the son of black sharecroppers, a regular in the juke joints of the Mississippi Delta. But on Nov. 23, 1936, Robert Johnson and Pablo Casals each walked into a room alone with their instruments. And on that day, each made recordings that would change music history.