This study examines the inclusion of the Ikhwān al-Ṣafāʾ’s epistles 'On Music' and 'On Proportions' (Epistles 5 and 6) under a single title, 'Kitāb al-mūsīqā (Book of Music)', in the Berlin composite manuscript Or. Oct. 350, a fifteenth-century Judeo-Arabic compilation from the Iberian Peninsula devoted to medicine. The 'Kitāb al-mūsīqā' (fol. 28v–30v) also contains a short passage on the musical nature of the pulse that explicitly refers to Ibn Sīnā (Avicenna) – a section absent from the known Arabic corpus of the 'Epistles'. Through an examination of the codicological and textual structure of the manuscript, the study investigates how the transmission of ʿilm al-mūsīqī (the science of music) within a Judeo-Arabic medical framework reflects a broader understanding of ratios and proportions as principles connecting musical, mathematical, and physiological knowledge. It suggests that such an understanding preserves the original theoretical sense of 'mūsīqī', later echoed in its European reception under the aegis of al-Fārābī.