In this paper, a conjugate spectral problem and biorthogonality conditions for the problem of extended plates of variable thickness are constructed. A technique for solving problems and numerical results on the propagation of waves in infinite extended viscoelastic plates of variable thickness is described. The viscous properties of the material are taken into account using the Voltaire integral operator. The investigation is carried out within the framework of the spatial theory of viscoelasticity. The technique is based on the separation of spatial variables and the formulation of a boundary value problem for Eigen values which are solved by the Godunov orthogonal sweep method and the Muller method. Numerical values of the real and imaginary parts of the phase velocity are obtained depending on the wave numbers. In this case, the coincidence of numerical results with known data is obtained.