Relaxation parts of complex velocity coefficients in the ordered phase of nematic liquid crystals are calculated. They are caused by the order parameter relaxation and fluctuations, and the relaxation of vector director fluctuations as well. Temperature and frequency dependencies of Leslie coefficients, anisotropy and dispersion of the sound velocity are anlayzed. It is discovered that the angle dependence of the sound velocity may be explained taking into account relaxation mechanisms. The sound velocity and Leslie coefficients calculated are compared with the experimental values.