The sounds come from the auditory cortex part of the brain; that has been proven by brain imaging studies. The most prevalent theory is that some disruption in the communication between the ears and the auditory cortex (loud noise, ear infection, age related hearing loss, etc.) sets off a standing pattern of hyperactivity of neurons in the auditory cortex. Why the brain does not naturally resolve this imbalance and hyperactivity is a real mystery, and that is probably where a cure will come from, if ever.
Other than this explanation, I would agree that it is Satan's symphony.