The Firebird (1934)

Prohibited from seeing her actor sweetheart Herman Brandt by her tyrannical parents, sweet young Vienesse lass Mariette defies authority by regularly visiting Brandt's downstairs apartment. The lovers' signal is a song called "The Firebird," which Brandt sings whenever he wants Mariette to visit him. When the actor is murdered, poor Mariette and her parents are prime suspects. But the truth is a bit more complicated than that, involving as it does a haughty aristocrat, a powerful diplomat and a most unusual "candid camera" device.