Multiferroic oxide thin films continue to receive intensive research interest due to their excellent combination of properties that are related to the ferroic orders. In this paper, BiFeO<sub>3 </sub>thin films are fabricated on (001) Si substrates at varying ad-atom energy to establish the role of microstructure and stress evolution on their elastic properties. Stress evolution in the films is accomplished through incorporation of argon ions during in-situ film growth. The dynamics of the propagation of surface acoustic waves on BiFeO<sub>3</sub> thin films of varying microstructure and stress is investigated using surface Brillouin scattering (SBS). The film substrate configuration is of a slow on fast configuration that supports Rayleigh surface acoustic waves (RSAW) and Sezawa waves. Hence the phonon velocity dispersion curves were determined from the frequency shifts of both sets of the waves in the discrete region at constant incident angle of 71 deg. Using the inverse problem the elastic constant for isotropic film systems have been determined with the results showing that Ar<sup>+</sup> incorporation decreases the bulk moduli and increases the shear moduli by 50%.