The present paper is concerned with the propagation of plane waves in a porous medium composed of two solids and two immiscible fluids. Using the principal of virtual complementary work, the expression for complementary strain energy density has been derived and the stress-strain relations for isotropic porous medium are obtained. The capillary pressure arising due to the pressure difference at the interface of two fluid phases has been taken into account, and the solid phases are assumed to be weakly coupled. The idea of compressibility tests is used to find values of the coefficients occurring into the stress-strain relations in terms of the physical properties of individual solid and fluid phases. The equations of motion are obtained using Lagrangian approach, and it has been shown that there may exist four compressional and two shear waves in the medium. Phase speeds of these waves are computed numerically and their dependence on frequency has been depicted graphically.