Die Antwort kennt nur der Wind (1974)

The yacht of millionaire Hellmann explodes in the Mediterranean Sea, with him and 8 men crew on board. Insurance agent Lucas investigates at the Cote D'Azur, because there's the suspicion that it could have been suicide. He finds out that Hellmann's villa was monitored, the tapes also contain information about the explosion. He uses them for blackmail.