Some critics argue that an opera’s stage directions are never reflected in its music.
Conclusion (implied)
An opera's stage directions are sometimes reflected in its music.
Intermediate Conclusion (Hence)
A change of scenery, which is a basic stage direction, can be reflected in the music, so other operatic stage directions can also be reflected.
Evidence
In some of Mozart's funny operas, there are squeaky violin noises that Mozart clearly meant to echo the sounds that are made while scenery is changing (set pieces are often on rolling wooden frames, so they might squeak ... big backdrops are lowered or raised from the rafters via rope and pully).
Evaluate / Goal
They're asking us about the Mozart premise, and asking what claim it was offered in support of. We could say that it was directly supporting the intermediate conclusion that , or that it was ultimately supporting the author's rebuttal to the position in the first sentence; our author is saying .