This is an interesting paper, recently published, that outlines research being done into a microphone that measures the change in air pressure at an interface between a lens and air, as changes in refractive index of the air as the pressure changes...
The Acoustical Society of Japan:
http://www.jstage.jst.go.jp/article/ast/29/4/283/_pdfAbstract: A new microphone, which uses the optical total reflection on the boundary surface
between glass and air, has been proposed. The intensity of reflected light changes by the sound
pressure, as the fluctuation of air density due to sound changes the refractive index of air and the
change in the refractive index changes the angle of total reflection. The sound pressure can be
measured as the change in the intensity of reflected light. In this paper we investigate theoretically the
sound pressure sensitivity of the proposed microphone in certain types of the shape of curved boundary
surface. As the sensitivity can be improved considerably by employing a plane wave light source, such
as laser light, and the curved surface with a large radius of curvature, the microphone is expected to be
practically used. The microphone detects the sound pressure without mechanical vibration and can be
made small, so its upper limit in the frequency range is very high in principle.
...so the potential is ruler-flat, 1Hz-100kHz frequency response in something the size of a pen?
Previous paper by the same authors...
http://www.jstage.jst.go.jp/article/ast/25/1/45/_pdfdigifish