In state security and forensic contexts, auditory surveillance through wiretapping and sound recording is as old as the technologies that have enabled it since the 1890s. Historical and critical studies of systematic eavesdropping have commonly focused on the act of taping, the decoding of encrypted messages, and the sociopolitical contexts and consequences of these activities. This project, however, centers on the history of research into the recordings’ sonic features, such as the detection of signals amidst noise, the identification of speakers and non-speech sounds, and the sonic verification of the tapes’ authenticity. It does so by studying two settings: the Ministerium für Staatssicherheit (Stasi) in the German Democratic Republic and its research program on analyzing recorded sound (1966–1989); and the field of audio forensics in the United States. While US audio forensics started during World War II with speaker identification, its scope expanded significantly in the 1960s. Which characteristics of sound did German and American experts consider relevant, and why? What techniques and technologies of auditory analysis and mechanical visual analysis of sound did they use? How did audio forensics and state security programs inform each other, also across the East–West divide? And how did these activities affect the use of sonic skills in the sciences more widely? Methodologically, the project draws on the analysis of archival documents, journals of audio engineering and forensics, and sonic elicitation interviews with experts. Theoretically, it aims to link STS theories on “sonic skills” with sound studies work on “acousmatic sound.” And societally, it aims to put recent debates about auditory surveillance into historical context by working with radio makers on a radio feature focusing on the past of audio forensics.