An implantable microphone comprises a hermetically-sealed, enclosed volume and an electret member and back plate disposed with a space therebetween and capacitively coupleable to provide an output signal indicative of acoustic signals incident upon at least one of the electret member and back plate. The back plate may be disposed to define a peripheral portion of the enclosed volume, e.g., the back plate may be defined as part of a flexible diaphragm that receives external acoustic signals. Vents may be provided to fluidly interconnect first and second portions of the enclosed volume that are located on first and second sides of the electret member. In another embodiment, the electret member may be flexible and spaced relative to a flexible outer diaphragm.