  The elastic modulus $c_{44}$ of a single crystal of the Weyl semimetal TaAs
was investigated by measuring relative changes in the sound velocity under
application of a magnetic field up to 10 T. Using an ultrasonic pulsed-echo
technique, we studied the shear response of the crystal when the angle between
the sound wave propagation and the magnetic field is changed. We observe a
broken tetragonal symmetry at fields above 6 T, an anisotropy that is likely
related to a longitudinal negative magnetoresistance and therefore might
provide evidence of the chiral anomaly, one of the main topological signatures
of this class of materials. We also observe quantum oscillations in the sound
velocity whose frequencies vary with magnetic field orientation. A fan diagram
of Landau level indices reveals topological and trivial Berry phases, depending
on the field orientation, indicating a sensitivity to different Fermi surface
pockets that do or do not enclose Weyl nodes respectively.
