Do you like horror films? If not, you might want to skip ahead, because this next section might get a little creepy. It shows how the separate hemispheres of the brain work as relates to singing.
When a patient has a life-threatening brain tumor in one half of the brain, sometimes the entire diseased hemisphere is surgically removed. Eewwwww! But the person is still alive, and that's good. So what happens?
Right brain in charge
According to Thomas R. Blakeslee, a lot of interesting things happen. In his book, The Right Brain: A New Understanding of the Unconscious Mind and Its Creative Powers (1983, pp. 152-55), he gives the following collection of descriptions of several different patients. If the left hemisphere is removed,
"[t]he patient typically loses all sense of language except his ability to curse . . . In spite of a sizable comprehension vocabulary, [the patient] continued to be unable to write or speak language . . . One dramatic exception was his ability to sing entire familiar songs . . . It appears that the right brain can store lengthy automatic sequences of speech (like song lyrics) but cannot create a long sequence of words . . . (But) his intonation and ability to recognize intonation is even better than in the normal state."
Left brain in charge
What happens when the right hemisphere is removed and the left brain works by itself?
"The patient becomes more talkative . . . his vocabulary becomes richer. At the same time, his intonation is less expressive: it is monotonous, colorless, and dull. His voice itself acquires a kind of nasal twang, or becomes unnatural, as though the patient were barking . . . The left hemisphere is unable to detect things such as anger, playfulness, or enthusiasm as communicated by intonation of the voice . . . Another defect that occurs is the inability to sing or recognize well-known tunes. When asked to hum along with music, the patient will generally hum the wrong notes and eventually end up just tapping out the rhythm without the melody." (ibid., pp. 157-58)
So the right brain is in charge of intonation, melody, pitch, expressiveness of sound, recognition of intonation in others' voices, and storage of lengthy automatic sequences of speech (like learning a song as a string of lyrics by rote memorization).
So the beauty of the singing voice clearly lives in the right brain. So does the melody, pitch, and one kind of memory for songs. But you must also have the left brain's ability to make the song sound like it is spontaneously spoken out of your own thoughts and to give all the lyrics their full, rich meaning. And you've got to get the rhythm right too. This requires a lot of constant communication between the two hemispheres in the very moments that you are singing.
Being tone-deaf is often caused by staying focused on the left brain and not accepting any input from the right brain. This can be the result of being educated and working in a world like ours, where the left brain is considered superior. When the left brain seizes control and the pitch flees from you during performance, it can feel like you've been dropped into a horror film!
Another way to look at the functioning of the two hemispheres is to explore what happens when a patient has epileptic seizures. The seizures are like electrical storms that spread from one hemisphere of the brain to the other. If the seizures get bad enough, they can interfere with the patient's everyday life. When medication fails to control them, doctors may, as a last resort, cut the nerves (corpus callosum) connecting the brain's hemispheres. This works pretty well to stop the seizures, and after the surgery, many patients appear normal in most circumstances. Their left brain takes cues from the right brain's actions outside the body to use the knowledge that is functioning in the right brain. But sometimes the hemispheres disagree. Then you can see the argument outside the body, as in this story from Blakeslee's book about the first split brain patient.
"[W]hile the patient was dressing and trying to pull on his trousers, the left hand might start to work against the right to pull the trousers down on that side. Or, the left hand, after just helping to tie the belt of the patient's robe, might go ahead on its own to untie the completed knot, whereupon the right hand would have to supervene again to retie it." (ibid., p. 120)
Geez, this creeps me out! When there is not internal communication between the hemispheres, the right brain can have an external, independent agenda of its own. And the corresponding left side can start acting like "Thing," the live severed hand from The Addams Family musical.
In conclusion, "horror" equals no communications between the brain's hemispheres. "Peace, success, and stardom" equals a great deal of communication between the brains' hemispheres.