In this paper we consider the propagation of linear transverse acoustic waves in isotropic media in which mechanical relaxation phenomena occur. It is assumed that the irreversible mechanical processes in the medium are due to viscosity and to changes in a tensorial internal variable and that these processes can be described with the aid of non-equilibrium thermodynamics. The viscous flow phenomenon is analogous to the viscous flow of ordinary fluids. In particular we investigate the velocity and attenuation of the waves and we consider the limiting cases of waves high and low frequencies. For high frequencies we obtain expressions for the phase velocity and attenuation which are analogous to those for viscous fluids and it is seen that the tensorial internal variables does not influence the propagation of waves. If the frequencies are sufficiently low, the expression for the phase velocity is analogous to the expression for the phase velocity in purely elastic media, while the damping is influenced by both irreversible phenomena and the imaginary part of the complex wave number is proportional to the square of the frequency.