Solidly Mounted Resonators (SMR) are alternatives to membrane resonators to realize Bulk Acoustic Wave (BAW) filters for RF communication systems. SMR decoupling from substrate, which is realized with a multi-layered Bragg reflector, has a major influence on resonator performance (quality factor, spurious modes). A finite element-boundary element (FEM-BEM) method is presented for two-dimensional SMR simulation. The substrate is described by an elastodynamic half-space Green function which is discretized and implemented in the ATILA finite element software. Results are presented for an aluminum nitride resonator with molybdenum electrodes operating at 2.14 GHz and decoupled from the substrate by a tungsten (W)/silicon oxide (SiO2) or a silicon nitride (SiN)/silicon oxycarbide (SiOC) Bragg reflector. When compared to one-dimensional Mason's model results, FEM-BEM computed values show a decrease of the quality factor and of the effective coupling coefficient. Lowered quality factors at parallel resonance are attributed to elastodynamic radiation into the substrate at the vicinity of the electrode boundary