Amusia is a neurological disorder specifically involving music. An amusic cannot understand the pitch of the notes or distinguish one melody from another, he cannot understand when it is out of tune and when not, nor does he know how to notice the out of tune of the others - specifically, he cannot identify variations in pitch less than two semitones. In severe cases, they are unable to hear music or find it irritating and unpleasant, and would not be able to distinguish a chant from a national anthem or a symphony.
Citing a historical example, it seems that Che Guevara was unable to distinguish any musical genre. The description of these symptoms alone would seem to support the theory of musical specialization of a brain module.