[en] The calculations and the results that predicted a fast-sound mode in low- as well as high-density binary fluid mixtures are presented in detail. Kinetic theory was used, which allowed a detailed discussion of the microscopic dynamics of the fluid in terms of eigenmodes. The Bhatnagar-Gross-Krook approximation method extended to binary mixtures was employed to solve the appropriate kinetic equation and to obtain approximate eigenmodes of the mixture. The fast-sound mode appears far beyond the hydrodynamic regime as either an extension to higher wave numbers of the hydrodynamic sound eigenmode or as a kinetic eigenmode. Some recent and possible future experiments are discussed