The Shadow of the Giant 2000 one on netflix

Capt. Eugenio arrives in a remote castle to take charge of a unit guarding a nameless prisoner. No one knows much about the inmate as he is held in a forlorn tower. When the captain’s wife Adele tries to shake her melancholia by playing the piano, she finds — much to her surprise — that she’s being accompanied by violin music coming from the prison cell.