Electret microphones dedicated to consumer electronics and medical applications (hearing aids) have reached the miniaturization limits. Since the release of the first microphone based on silicon micromachining, electret microphones are constantly replaced by MEMS microphones. Regardless of the transduction principle (capacitive, piezoresistive, piezoelectric, optical), all of the MEMS microphones reported in the state of the art literature are based on a membrane deflecting out of the plane of the base wafer. On the contrary, the novel microphone architecture that is developed in the frame of the ANR MADNEMS project uses micro beams that deflect in the plane of the base wafer. The presented microphone profits of the well known technological platform developed at CEA LETI that integrates micro and nanofabrication to deliver high- performance MEMS sensors. Transduction is achieved by piezoresistive nano gauges integrated in the microsystem, arranged in a Wheatstone bridge and attached to micro beams. Acoustic pressure fluctuations lead to the deflection of the micro beams which produces a stress concentration in the nano gauges. Such architecture enables us to reduce the surface of the deflecting element and leads to a microphone with a smaller footprint that preserves at the same time high performance. Accurate simulations of the discussed transducer couple acoustic, mechanic and electric behavior of the system. Due to micrometric dimensions of acoustic vents, thermal and viscous boundary layers have to be taken into account. Additionally the influence of backspace volume on pressure response has to be examined. The paper will initially present general principle of operation and the technological process; then, the coupled microphone model will be briefly presented. Finally we will focus on the parametric tests (technological process properties) and electromechanical test (mechanical properties of MEMS) of the microphones.