The market and the applications of micro-electromechanical systems (MEMS)-based microphones have been in continuous growth over the last decades. This article presents a promising acoustic-sensing technology that mixes the consolidated MEMS technology with an innovative optical transduction technique of acoustic signals. The proposed method allows to significantly reduce the intrinsic noise of the system and to increase its signal-to-noise ratio (SNR). The designed digital optical microphone reaches an SNR of 71.6 dBA in a <inline-formula> <tex-math notation="LaTeX">5{\times } 5{\times } 2 </tex-math></inline-formula> mm3 package with an output sensitivity of -21 dBFS/Pa. This article describes each section of the system-in-package (SiP) microphone, starting from the physics behind the transduction mechanism, covering the application-specified integrated circuit (ASIC) and package design, and the optical stack structure. A final analysis of the obtained experimental results is provided and compared with the state-of-art reported in the literature.