  We study the in-plane acoustic phonons in twisted bilayer graphenes using the
effective continuum approach. We calculate the phonon modes by solving the
continuum equation of motion for infinitesimal vibration around the static
relaxed state with triangular domain structure. We find that the moir\'{e}
interlayer potential only affects the in-plane asymmetric modes, where the
original linear dispersion is broken down into miniphonon bands separated by
gaps, while the in-plane symmetric modes with their linear dispersion are
hardly affected. The phonon wave functions of asymmetric modes are regarded as
collective vibrations of the domain-wall network, and the low-energy phonon
band structure can be qualitatively described by an effective moir\'{e}-scale
lattice model.

Mesoscale and Nanoscale Physics