Previously reported inelastic X-ray scattering spectra of molecular liquid benzene were reanalyzed by a generalized Langevin formalism (GLF) with a memory function including a thermal and two viscoelastic relaxation processes. A new algorithm was containing a simple sparse modeling to smooth Q dependences of dynamical parameters used in the present GLF analysis. The obtained excitation energies of longitudinal acoustic phonons show a largely positive deviation from the hydrodynamic value by about 50%, the same as that by previous analysis and similar to existing results of several molecular liquids. Such a large deviation of dynamical sound velocity in liquid benzene is interpreted as extra energy-losses for terahertz phonons by vibrational and rotational motions of benzene molecules. The rates of the fast and slow viscoelastic relaxations in the memory function at low Q are undetectable and 0.4-0.8 ps, respectively. These can be explained by the vibrational and anisotropic rotational motions of benzene molecules. The microscopic kinematic longitudinal viscosity rapidly decreases with Q from a reasonable macroscopic value.