What is the nature of film sound? How does it change through time? How can film sound be conceptually defined? To address these issues, this work assumes the perspective of film preservation and presentation practices, describing the preservation of early sound systems, as well as the presentation of film sound in the spaces provided by film heritage institutions. The preservation and presentation practices analyzed bring different dimensions of film sound to the fore, such as its material, human, technological, institutional, experiential, and memorial dimensions. The definition of film sound is constructed around the concepts of material form, trace, and performance, which explain its transitory nature and its many facets.