We report a molecular dynamics (MD) study of collective dynamics in molten ${\text{Li}}_{4}\text{Tl}$ alloy, which was studied experimentally by inelastic neutron-scattering experiments and is supposed to have ``fast sound'' excitations because of a large ratio of components masses. Time-correlation functions obtained in MD simulations are analyzed by the generalized collective modes approach, that enables to obtain dispersion and damping of the different collective modes, and to estimate their contributions to the time-correlation functions and their corresponding dynamic-structure factors. We show, that there exists a transition in the contributions from the high- and low-frequency branches of collective excitations to the total dynamic-structure factor for wave numbers $k$ around $0.4\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$. Only the low-frequency hydrodynamic acoustic modes without any fast sound define the Brillouin peak in the long-wavelength range, while the high-frequency modes mainly contribute to the side peak of the total dynamic-structure factor for $kg0.4\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$.