Coherent acoustic phonons detected by time-domain Brillouin spectroscopy (TDBS) in a liquid medium are strongly attenuated when silica nanoparticles sediment in the liquid. The decrease in the optical signal is due to the strong acoustic scattering by silica nanoparticles whose size matches the wavelength of phonons at the frequency of the Brillouin shift. The control of the acoustic wave-front with nanoparticles thus strongly reduces the undesired Brillouin response of the liquid medium. This is demonstrated with a thin transparent film in water, mimicking a biological sample in a nutritive serum. While strong Brillouin scattering in the particle-free liquid hampers the measurement in the film, we show that TDBS can however be performed in a film of a thickness less than the optical wavelength when scatterers sediment in the liquid.