This article is an attempt to understand music theory from the perspective of written and sounding media. It examines three radically different music-theoretical practices, which operate with different forms of written notation and different musical instruments, and have surprisingly different purposes in mind: the monochord-based theory of Franchinus Gaffurius (1518), the siren-based theory of Wilhelm Opelt (1834) and the piano-and-score-based theory commonly practised in our age. The instruments used in these three music theories hold the key to a fuller understanding: they can be understood as ‘epistemic things’ – that is, in producing sounds, these objects simultaneously produce knowledge about music. From a media-archaeological perspective, I suggest, these three music-theoretical practices stand emblematically for Pythagorean, digital and textual approaches to music.