‘All dark,’ Dudley said hoarsely, shuddering. ‘Everything dark. And then I h-heard ... things. Inside m-my head...’
Uncle Vernon and Aunt Petunia exchanged looks of utter horror. If their least favourite thing in the world was magic, closely followed by neighbours who cheated more than they did on the hosepipe ban, people who heard voices were definitely in the bottom ten. They obviously thought Dudley was losing his mind.
Harry Potter and the Order of the Phoenix
As I understand, "people who heard voices were definitely in the bottom ten" is saying that Uncle Vernon and Aunt Petunia disliked people who heard voices(because it's in the bottom ten of their favourite list). But I'm not sure how it's relevant to the context. Maybe, they thought Dudley was losing his mind because he heard voices? What's the sense of it here?