At the crystallographic transition (14°K) in DyVO4 the c44 and c55 modes show continuous and opposite changes of the order of 1/2% in the sound velocity. The longitudinal mode c33 exhibits a change which is more than an order of magnitude smaller. The results can be explained using higher order Jahn-Teller energy terms. This enables us to determine the temperature dependence of the order parameter. Upon applying a magnetic field one can demonstrate the influence of crystallographic domains on the shear sound mode. Additional effects due to the Zeemann splitting of the electronic levels are seen.