A sound sensor implantable in a recipient of a hearing device. The sound sensor comprises a biocompatible housing comprising a cavity having an opening at a first end of the housing, and a membrane attached to the housing so as to hermetically seal the opening. The membrane vibrates in response to vibration of at least on a structure of the recipient's ear and fluid within one of the recipient's body cavities. The sound sensor also comprises a vibrational sensor disposed in the housing configured to detect vibrations of the membrane, and configured to generate signals representative of the detected vibrations; and a transmitter configured to provide the generated signals to one or more other components implanted in the recipient.