In this paper we present a hologram-based real-time system which allows the user to interact with music according to the diffractive light from a hologram. The system synthesizes audio with different pitches and tempi in accordance with the variations of the light brightness from a dot matrix hologram, which is composed of many gratings with varied orientations and pitches. Music data are encoded in a 2-D barcode on the hologram. The hologram possesses the "image switching" feature for real-time detection and this feature can affect the tempo of the synthesized music. The results show that this interactive system successfully decodes the barcode and synthesizes audio according to the variations of the diffractive lights from the hologram. This interactive video/audio system has many potential applications to creative music performance, interactive art presentation, e-learning, game development and anti-counterfeiting technology.