The effect of ultra-high-frequency sound on the magnetic moment of an antiferromagnet with strong magnetoelastic interaction is studied. Quasiphonons of frequency approximately 1 GHz were excited by an external piezotransducer or by a microwave field. At liquid nitrogen temperatures the total magnetic moment of the sample is found to decrease with increasing number of quasiphonons, whereas at liquid helium temperatures the magnetic moment increases. This increase can be considered as an effect of dynamic polarization induced by hypersound excitation, which is observed for the first time. The dynamic polarization can be explained in terms of a three-particle process: a quasiphonon and a quasimagnon are annihilated and a new quasimagnon with smaller magnetic moment is created.