Rayleigh-Brillouin scattering has been studied in a gas mixture of He and Xe at various concentrations. Pronounced broadening and frequency pulling of the Brillouin components is observed with increasing He concentration due to a strong coupling between the sound wave and concentration diffusion modes in the mixture. The complete solution of the hydrodynamic equations accurately describes the observed spectra and reveals the importance of the coupling between the fluctuating thermodynamic variables. A simplified analysis of the interaction between the pressure and concentration fluctuations is given that describes the main features of the observed spectral behavior