The Shadow of the Giant(2000)

Original title: L'ombra del gigante

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.