Article: Musical Hallucinations

Neuron Network Goes Awry, and Brain Becomes an IPod – New York Times
The results support recent work by neuroscientists indicating that our brains use special networks of neurons to perceive music. When sounds first enter the brain, they activate a region near the ears called the primary auditory cortex that starts processing sounds at their most basic level. The auditory cortex then passes on signals of its own to other regions, which can recognize more complex features of music, like rhythm, key changes and melody.
For most people, these spontaneous signals may produce nothing more than a song that is hard to get out of the head. But the constant stream of information coming in from the ears suppresses the false music.
Dr. Griffith proposes that deafness cuts off this information stream. And in a few deaf people the music-seeking circuits go into overdrive. They hear music all the time, and not just the vague murmurs of a stuck tune. It becomes as real as any normal perception.