The equations of motion of nematic liquid crystals are complemented by the nonlinear Langevin equation for the smectic order parameter fluctuations. The nonlinear terms of this equation describe coupling of the order parameter to velocity and density. An expression for the reactive part of the stress tensor is obtained. The complex fluctuation inputs into viscosity coefficients are calculated in the frame of the Landau–de Gennes model near the nematic–smectic-A (N-A) phase transition. The sound velocity dispersion and the attenuation dependences on temperature, frequency, and the direction of the sound wave propagation are calculated. The results obtained are sensitive to values of two critical exponents for correlation lengths.