Audio technologies that allowed eavesdropping on private conversations were a key tool in Cold War–era surveillance practices. In 1975, in the midst of the Cold War, a criminal police agency called the Fonoscopy Department was established in Czechoslovakia's capital, Prague, to explore the forensic potential of sound analysis for speaker identification. This article reveals for the first time that, aside from the well-known Czechoslovak secret police's wiretapping and eavesdropping activities, an independent government agency engaged in forensic fonoscopy, developing sound-based expertise. Examining the department's practices challenges the notion of mechanical and visually grounded objectivity to show how forensic science negotiated objective knowledge at the intersection of aural analysis and visualization technologies. More generally, the article contributes to debates on utilizing "sonic skills" to produce knowledge and evidence for security and legal purposes.