The Life of Others, a 2006 thriller film, informed a wide audience about practices of eavesdropping on telephone calls and private conversations in the German Democratic Republic (GDR). By the end of the 1980s, GDR’s Ministry of State Security (Stasi) was able to execute 20,000 wiretapping and eavesdropping actions in East-Berlin at one and the same time. This frantic activity required the use of automatic recording technologies and a voice data bank. From the 1960s onward, the Stasi also felt forced to work on the issue of how to identify speakers by the characteristics of their recorded voice, as its employees did not always know whom they were listening to. Yet how did these employees make sense of the taped voices? Which aspects of the recorded sounds did they consider relevant for their diagnostic purposes? How did they dissect these sounds, and how did listening become a legitimate way of knowing in this context? These questions will not only be answered on the basis of Stasi archives and by situating the Stasi activities in the Cold War, but also by comparing the Stasi strategies with twentieth-century “sonic skills” in science, engineering and medicine more widely.