I've just read Stephen Fry's latest autobiographical tome in which he relates how he resorted to Hypnotism to get him through a comic sketch where he had to sing.
He claims that he still thinks of himself as tone-deaf, and Paul McCartney agrees with him!
However his tone-deafness stems from an incident at school where he was advised to mime and the hypnotism did work for the one occasion where he was told that after a cue sentence he would be able to sing in tune.