Thanks for Listening (1937)

Homer Tompkins, a trusting young musician with aspirations of becoming a radio star, is installed in a fine set of offices in Reno by a quartet of con artists who have bugged rooms and suites, with the title of "professional listener", and he passes off what he hears to his employers, Lulu, Trixie, Maurice and Champ and they use the information for blackmail purposes. They also so involve his sweetheart, Toots, that he can not withdraw even when he finally learns he is being used as a tool.