Fermi liquids respond differently to perturbations depending on whether their frequency is higher (collisionlessregime) or lower (hydrodynamic regime) than the interparticle collision rate. This results in a different phasevelocity between the collisionless zero sound and the hydrodynamic first sound. We performed terahertz pho-tocurrent nanoscopy measurements on graphene devices, with a metallic gate close to the graphene layer, toprobe the dispersion of propagating acoustic plasmons, the counterpart of sound modes in electronic Fermiliquids. We report the observation of a change in the plasmon phase velocity when the excitation frequencyapproaches the electron-electron collision rate that is compatible with the transition between the zero andthe first sound mode.