  The phonon spectra of twisted bilayer graphene (tBLG) are analyzed for a
series of 692 twisting angle values in the $[0,30{\degree}]$ range. The
evolution of the phonon bandstructure as a function of twist angle is examined
using a band unfolding scheme where the large number of phonon modes computed
at the $\Gamma$ point for the large moir\'e tBLG supercells are unfolded onto
the Brillouin Zone (BZ) of one of the two constituent layers. In addition to
changes to the low-frequency breathing and shear modes, a series of
well-defined side-bands around high-symmetry points of the extended BZ emerge
due to the twist angle-dependent structural relaxation. The results are
rationalized by introducing a nearly-free-phonon model that highlights the
central role played by solitons in the description of the new phonon branches,
which are particularly pronounced for structures with small twist angles, below
a buckling angle $\theta_{\rm B}\sim {3.75}{\degree}$.
