I really like the irony of these lines spoken by Olivia. Pythagoras's music of the sphere's theory proposes that planetary orbits produce sound based on the distance from the earth. According to the theory, we should be able to hear these musical sounds all the way on Earth, the center of the model. However, we are not able to. My understanding of these lines are that, while Olivia makes it plain that there is no harmony between her and Orsino, she's also not receiving the message from Viola as Cesario; there may be a kind of harmony there, but it's being refuted by Viola and therefore, this unrequited love makes Olivia feel as though there is no harmony in the universe at all.