  Magnetochiral effect (MChE) of phonons, a nonreciprocal acoustic property
arising due to the symmetry principles, is demonstrated in a chiral-lattice
ferrimagnet Cu$_2$OSeO$_3$. Our high-resolution ultrasound experiments reveal
that the sound velocity differs for parallel and antiparallel propagation with
respect to the external magnetic field.The sign of the nonreciprocity depends
on the chirality of the crystal in accordance with the selection rule of the
MChE. The nonreciprocity is enhanced below the magnetic ordering temperature
and at higher ultrasound frequencies, which is quantitatively explained by a
proposed magnon-phonon hybridization mechanism.
