In this article I explore the relationship of theories of sound reproduction formulated in the decades between the two world wars with the studio practice of Pierre Schaeffer. I argue that the 1920s-1930s was a period of significance for sound-based arts, and compare it to analogous defining moments in cinema and art photography. After examining the legacy of this period, I turn to one specific moment from Schaeffer's early studio experiments with musique concrete in April 1948, showing how the theories of sound reproduction formed in the earlier time period informed practical decisions in Schaeffer's working methods at a critical time when his ideas about the sound object were forming. Schaeffer's studio practice and, to an extent, his theories of listening thus carry traces of this prior sonic culture. Considering the decisive influence of Schaeffer's writings and teaching on later generations and developments in electroacoustic music, I speculate on the proliferation of these ideas beyond Schaeffer's immediate circle, focusing in particular on soundscape composition. The title of this article is a reference to James Lastra's invisible auditor', a term he coined to characterise the approach to sound reproduction discussed in this article (Lastra 2000: 159).