The objective of this study is to develop an interactive system that can synthesize audio in accordance with the variations in brightness of the diffracted light from a dot matrix hologram, which is composed of many gratings with varied orientations and pitches. The diffracted light is converted into audio signals with different pitches and tempos. In order to improve the accuracy of timing of the synthesized audio from the hologram, the fundamental concept of diffraction and signal processing are used. The results show that this interactive system successfully synthesizes audio according to the variations of the diffractive lights from the hologram. This real-time interactive system has great potential to creative art presentation, e-learning, game development and anti-counterfeiting applications.