Note to Twelfth Night, 5.1.110, "As howling after music"

When Olivia says this, the man to whom she believes herself to be betrothed is standing at Orsino's side. Therefore her comparison may contain a message for him, as well as for Orsino. Olivia may be reminding Cesario that his vow to marry her was music to her ears. If she does intend to deliver such a message, it adds to the comic confusion, as not only is Cesario really Viola in disguise, but Olivia is really betrothed to Viola's twin, Sebastian.)

Anne Hathaway as Viola and Sebastian/Cesario