How does Terrence McNally use imagery in Master Class?

Obviously, in a play about a legendary opera diva, music is of central importance. Not only are two Callas recordings played, but the tenor and soprano sing arias on stage (the latter does not complete hers). The centrality of singing, and the tragedy of Callas, whose voice deserted her at a comparatively young age, is forcefully made in the only line of music that Maria herself sings in the entire play. This comes midway through act two, and it is the opening of Lady Macbeth's aria, after she has read the letter. The stage directions read, "What comes out is a cracked and broken thing. A voice in ruins. It is a terrible moment." The audience is thus given a contrast to the glorious voice on the recordings and so becomes aware of its fragility—as well as the tragic vulnerability of the character on stage to whom the voice belongs.