This chapter considers how the blurring of sound design and music, often seen as a feature of contemporary cinema, developed within earlier forms of cinematic expression. Focusing on British documentary cinema, Andy Birtwistle examines innovative uses of sound in documentary filmmaking, paying particular attention to forms of cinematic musique concrète that emerged following early experimentation with film sound technology in the 1930s. Considering key works by directors Basil Wright and Geoffrey Jones, and composers Walter Leigh, Daphne Oram and Tristram Cary, the chapter considers the ways in which filmmakers and composers working in documentary have manipulated and organized recorded sound, producing forms of electracoustic practice that prefigure and parallel the radical developments that took place in modern music after World War II.