Skin could also be used as a sounding head as in a banjo. One of the versions I have heard has only three strings for the harp (made of the hair) which is not very many--the smallest listed in the Lark in the Morning catalog has 19 strings. Incidentally they also list a Kora--an African harp which has a gord belly, skin head, and gut strings. This would be close to the most ancient form of a harp, which has no pillar (therefore the strings can't be strung very tightly). I guess if I were going to construct a harp from body parts (which I'm not!) I would use the skull as the belly, skin as the sounding head, the largest rib as the neck, and either intestine or sinew as the strings. The breastbone wouldn't be very useful. The author probably chose that through a combination of ignorance of anatomy and because it rhymes with "stone" (i.e. "He made a harp of her breastbone, whose sound would melt a heart of stone").
Jon, also grosser than he wants to be.