The dynamic structure factor, S(Q,omega), of liquid benzene was measured by meV-resolved inelastic X-ray scattering (IXS) and analyzed using a generalized Langevin model with a memory function including fast, mu-relaxation and slow, structural, alpha-relaxation. The model well reproduced the experimental S(Q,omega) of liquid benzene. The dispersion relation of the collective excitation energy yields the high-frequency sound velocity for liquid benzene as related to the alpha-relaxation. The ratio of the high-frequency to the adiabatic sound velocity is approximately 1.5, larger to that of carbon tetrachloride and smaller than those of methanol and water, reflecting the nature of intermolecular interactions. (C) 2017 Elsevier B.V. All rights reserved.