The Biot theory for sound propagation in porous media has been specialized to suspensions of particles and compared with an earlier model based on scattering theory, for sound velocity and viscous attenuation in suspensions. It is shown that the Biot theory gives fairly correct predictions of attenuation as a function of concentration, whereas the suspension model fails when the concentration exceeds a few percent. The reason for this is discussed and a modification to the suspension theory is proposed. This theory has been applied to study viscous attenuation as a function of frequency and it is shown that viscous attenuation may have an almost linear dependence on frequency when account is taken of distributed grain sizes.