In Aristotle's 6 elements of a play, whats the difference between language/diction and the music?

diction actually means the kind of language used including the proper kind of verse forms, Aristotle said everybody knows what music is and so he didn't define it. All kinds of scholarly guesses have been made, from the poetry of the speeches, to assuming the lines were sung or chanted (which is why opera was created in the Italian Renaissance). So no one really knows. Could be there were musicians who accompanied the plays on instruments.