This refers to Philomela. In Greek legend she was an Athenian princess and sister of Procne, who married Tereus. Tereus violently deflowered Philomela and cut out her tongue to prevent her from accusing him. Philomela, however, embroidered her tale on a swatch of cloth (what a collectors’ item that would be) and sent it to her sister. Procne’s revenge was to serve Tereus the flesh of their son, and then the sisters fled. Tereus gave chase, but the sisters were turned into birds by compassionate gods. Philomela flew away as a nightingale but, according to some versions of the tale, she never regained her tongue. That would more than explain why her song was soft. In the big transformation scene a better-organized god would have allowed her to regain her tongue and even possibly her virginity.​

Soft the song of Philomel
Act II