This article presents the modeling of a MEMS microphone with an original architecture formed of mechanical structures moving in the plane of the substrate. On the contrary of most microphones generally constituted of an oscillating membrane, some transducers developed by the CEA-LETI with M&NEMS technology use micro beams moving in the plane of the silicon wafer under the effect of an acoustical wave. These micro-structures are connected to the substrate by flexible micro-hinge and strain silicon nano-gauge producing a variation in resistance by piezoresistive effect. After the description of the design and functioning of the microphone under study, the vibroacoustic model of the fluid-structure coupling is presented. Considering the dimensions of the MEMS transducer close to the thermal and viscous boundary layers thicknesses, this model has to include diffusion phenomena. The model is discretized using the finite element method and the weak formulation is implemented using COMSOL Multiphysics® software. The pressure sensitivity of the microphone is calculated and compared with an analytical lumped model to asses the numerical model. Pressure and velocity fileds are also computed. Solutions of simulations are interpreted by focusing on phenomena influencing the sensitivity of this novel sensor design. In particular, the influence of the geometry and the role of the different part of the transducer (back cavity, mechanical structures) are studied.