Here, we use stable droplets that co-host equatorial acoustical and optical resonances phase-matched to enable the exchange of energy and momentum between sound and light. Here, we use stable droplets that co-host equatorial acoustical and optical resonances phase-matched to enable the exchange of energy and momentum between sound and light. Relying on free-space visible laser excitation of multiple whispering-gallery modes, we harness a triple-resonant forward Brillouin scattering to stimulate opto-mechanical surface waves. Nonlinear amplification of droplet vibrations in the 60-70 MHz range is realized with spectral narrowing beyond the limit of material loss, thereby activating the droplet as hypersound-laser emitter.