Kinetic equations are derived for two kinds of gross variables of nematics in the isotropic phase, i.e., the independent components of orientational tensor order parameter and the transverse local momentum density, using Kawasaki’s mode-coupling theory. Based on them, the time correlation function and the relaxation rate for the critical fluctuations of the order parameter is obtained. The latter result explains why the conventional Landau–de Gennes theory is valid for the usual experiments like flow birefringence and light scattering. The sound attenuation coefficient due to the critical fluctuations of the order parameter is formulated by taking account of the mode-coupling effects between the two kinds of gross variables. The result is compared with the available experimental data with good improvement upon previous conventional theory.