In this work, an original finite element modeling to investigate the effects of a viscoelastic layer on the sound transmission through double-wall sandwich panels is presented. This formulation is obtained from a coupled fluid− structure variational principle taking into account the frequency dependence of the viscoelastic material. The resolution approach is based on a reduced order model generated by a modal projection technique. The sound insulation of the panels is evaluated by computing its sound transmission factor using Rayleigh integral method. An efficient and inexpensive finite element for a sandwich plate with viscoelastic core is developed. Various results are presented in order to validate and illustrate the efficiency of the proposed finite element formulations.