“I do not think you hear what you think you hear.”
Not a relativist thing: It is not saying that everyone hears what they want to hear, quite the opposite.
Not an idealist thing: It is not saying either that: a) the sound and the subject coincide, nor that b) the sound is some noumenal thing out there that the subject cannot know, but of which the subject can only know a phenomenal representation that is forever cut off from the noumenal.
A materialist thing: It is saying that the sounding object has so many aspects that no one subject can comprehend them all, and that relative weight will be given to certain aspects over others depending on the development of both the practical assumptions and the theoretical assumptions. Everything the listener hears is real, but the listener doesn’t hear everything that is real.
It is then saying that the (conscious) assumptions about what is important in the sound – which is real – may not coincide with the (unconscious) spontaneous response to the sound – which is also real. Of course, the important thing is uncover the not so spontaneous logic behind the apparently unconscious and spontaneous response: why really did I like that? was it really because of x and y things from the theory I have read? It may well be, but it may well not be. The subject is divided.
Massively important pedagogically.