  Topological phases have recently been realised in bosonic systems. The
associated boundary modes between regions of distinct topology have been used
to demonstrate robust waveguiding, protected from defects by the topology of
the surrounding bulk. A related type of topologically protected state that is
not propagating but is bound to a defect has not been demonstrated to date in a
bosonic setting. Here we demonstrate numerically and experimentally that an
acoustic mode can be topologically bound to a vortex fabricated in a
two-dimensional, Kekul\'e-distorted triangular acoustic lattice. Such lattice
realises an acoustic analogue of the Jackiw-Rossi mechanism that topologically
binds a bound state in a p-wave superconductor vortex. The acoustic bound state
is thus a bosonic \edit{\emph{analogue}} of Majorana bound state, \edit{where
the two valleys replace particle and hole components}. We numerically show that
it is topologically protected against arbitrary symmetry-preserving local
perturbations, and remains pinned to the Dirac frequency of the unperturbed
lattice regardless of parameter variations. We demonstrate our prediction
experimentally by 3D printing the vortex pattern in a plastic matrix and
measuring the spectrum of the acoustic response of the device. Despite
viscothermal losses, the measured topological resonance remains robust, with
its frequency closely matching our simulations.
