What did Arisottle think of the Pythagorean idea of "music of the spheres"?
Aristotle discusses celestial music in On the Heavens (De Caelo) book II part 9. He does not dispute (but also does not show much interest in) the claim that the distances between celestial bodies form harmonious ratios. What he does dispute is the Pythagorean theory that the movements of the stars subsequently produce actual music. He regards the Pythagorean theory, approvingly, as "gracious and original", as well as "melodious and poetical". It is, nevertheless, false.
From all this it is clear that the theory that the movement of the stars produces a harmony, i.e. that the sounds they make are concordant, in spite of the grace and originality with which it has been stated, is nevertheless untrue ... melodious and poetical as the theory is, it cannot be a true account of the facts.
If the stars produce music, why don't we hear it? The Pythagorean answer, according to Aristotle, was that we do hear it, but since we have always heard it, we can't tell it apart from true silence (which we've never really heard). This is a clever answer, and a possible one in isolation. The problem is that noises have objective effects, beside our subjective hearing. Loud noises shatter stuff, regardless of being or not being heard. And the music of the stars, if there was such, should have been loud enough to shatter everything around us.
Aristotle took this consideration to also support his view that (1) the stars do not move independently, but are fixed on transparent celestial spheres, and that (2) the celestial spheres move in a frictionless medium, an element finer than air.