Was he an animal, that music could move him so? He felt as if the way to the unknown nourishment he longed for were coming to light. (3.14)
It's unclear why musical taste should be indicative of Gregor's animal nature. Certainly there are mythic references to animals being swayed by music. Aren't humans equally enthralled by music? Perhaps Gregor is just noticing that he is moved by music in a way that he isn't moved by human language.