No songs, but in Ireland 7th sons of 7th sons were (are?) believed by many (some?) to have powers of healing.

In Terry Pratchett's discworld, the eighth son of an eighth is a wizard. Wizards are supposed remain celibate, to avoid any possibility of an eighth son of an eighth son of an eighth son. Such a one would be a sourcerer, (able to touch the source of magic), with enough power to destroy the entire world.