Using a realistic flexible molecule model of the fragile glass former orthoterphenyl, we calculate via molecular dynamics simulation the collective dynamic structure factor S(Q,omega), recently measured in this system by inelastic x-ray scattering. The comparison of the simulated and measured dynamic structure factor, and the study of the S(Q,omega) in an extended momentum (Q), frequency (omega), and temperature (T) range allows us (i) to conclude that the utilized molecular model gives rise to S(Q,omega) in agreement with the experimental data, for those thermodynamic states and Q values where the latter are available; (ii) to confirm the existence of a slope discontinuity on the T dependence of the sound velocity that, at finite Q's, takes place at a temperature T-x higher than the calorimetric glass transition temperature T-g; (iii) to find that the value of T-x is Q-dependent and that its Q -->0 limit is consistent with T-g. The latter finding is interpreted within the framework of the current description of the dynamics of supercooled liquids in terms of exploration of the potential energy landscape.