Sacred Hearts by Sarah Dunant

1570 in the Italian city of Ferrara.
Sixteen-year-old Serafina is fipped by her family from an illicit love affair and forced into the convent of Santa Caterina, renowned for its superb music. Serafina's one weapon is her glorious voice, but she refuses to sing.
Madonna Chiara, an abbess as fluent in politics as she is in prayer, finds her new charge has unleased a power play - rebellion, ecstasies and hysterias - within the convent.
However, watching over Serafina is Zuana, the sister in charge of the infirmary, who understands and might even challenge her incarceration.