When the outside world is silent, all sorts of sounds can come to mind: inner voices, snippets of past conversations, imaginary debates, and retrospective witticisms. What should we make of such sonic companions? This talk investigates a period when these and other aural phenomena prompted a far-reaching debate in the academic world. Through case studies from Paris, Vienna, Geneva, and Berlin, it shows that the identification of the auditory cortex in late nineteenth-century neuroanatomy affected numerous disciplines across the sciences and humanities. Each now created sound-related concepts that were central to its epistemological agenda. Ferdinand de Saussure interpreted the “acoustic image” as a key to human language, while Sigmund Freud approached the human psyche through the auditory unconscious, and similar processes unfolded across an enormous range of other disciplines. In an increasingly diverse academic landscape, with ever more specialized fields of research, auditory concepts thus worked in two ways: to draw epistemological distinctions, staking out a raison d’être for certain disciplines, and as a binding theme between disciplines. They did so not only by mapping out epistemic territory, but also by binding together theoretical with practical knowledge. The scholars and scientists presented in this talk responded creatively to the new cultures of music and audio communication arising around 1900, and to recording technologies that generated alternate modes of simulating, collecting, and comparing sound data. In turn, their auditory knowledge was applied in social, aesthetic, and industrial domains outside the academic realm. Through these conjunctions, this talk illuminates a moment in time that gave rise to many of the structures, theories, and fields of application underlying recent trends such as Sound Studies. More generally, it offers a deeper understanding of today’s second “sonic turn” in science and scholarship.