In 1942, Hitler paid a secret visit to Baron Carl Gustaf Emil Mannerheim, Marshal of Finland and Commander-in-Chief of the Finnish Defence Forces in honor of Mannerheim's 75th birthday.
Finnish broadcasting engineer Thor Damen covertly captured the first 11 minutes of their conversation, the only known recording of Hitler's normal conversational voice.
The SS realized that Damen was recording the conversation, and they immediately demanded to have it stopped. The SS were furious, but Yle was allowed to keep the tape hidden away, never to be opened. The tape was given to head of the state censors' office, Kustaa Vilkuna, returned to Yle in 1957, and made publicly available a few years later. It is the only known recording of Hitler speaking in an unofficial tone and one of the very few recordings in which Hitler may be heard delivering a narrative without raising his voice.
The conversation is about Hitler explaining the failure of Operation Barbarossa, Italian defeats in North Africa, Yugoslavia, and Albania, armaments in the Soviet Union, and Romanian petroleum wells.