  We theoretically study in-plane acoustic phonons of graphene/hexagonal boron
nitride moir\'e superlattice by using a continuum model. We demonstrate that
the original phonon bands of individual layers are strongly hybridized and
reconstructed into moir\'e phonon bands consisting of dispersive bands and flat
bands. The phonon band structure can be effectively described by a spring-mass
network model to simulate the motion of moir\'e domain walls, where the
flat-band modes are interpreted as vibrations of independent, decoupled
strings. We also show that the moir\'e phonon has angular momentum due to the
inversion symmetry breaking by hBN, with high amplitudes concentrated near
narrow gap region. Finally, we apply the same approach to twisted bilayer
graphene, and we find a notable difference between the origins of the flat-band
modes in G/hBN and TBG, reflecting distinct geometric structures of domain
pattern.
