This paper describes an acoustic sensor based on a single-crystal silicon diaphragm with an integrated Photonic Crystal (PC) mirror, whose deflection is measured in a Fabry-Perot resonator formed between the PC mirror and a gold coated single-mode fiber. The sensors are fabricated on standard silicon wafers, yielding monolithic, low-stress sensing diaphragms. The packaged sensor exhibits a minimum detectable pressure of 17μPa/√Hz in the 8.5kHz to 18kHz frequency range.