  Magnetic spinels (with chemical formula $AX_{2}$O$_{4}$, with $X$ a 3$d$
transition metal ion) that also have an orbital degeneracy are Jahn-Teller
active and hence possess a coupling between spin and lattice degrees of
freedom. At high temperatures, MgV$_{2}$O$_{4}$ is a cubic spinel based on
V$^{3+}$ ions with a spin $S$=1 and a triply degenerate orbital ground state. A
structural transition occurs at T$_{OO}$=63 K to an orbitally ordered phase
with a tetragonal unit cell followed by an antiferromagnetic transition of
T$_{N}$=42 K on cooling. We apply neutron spectroscopy in single crystals of
MgV$_{2}$O$_{4}$ to show an anomaly for intermediate wavevectors at T$_{OO}$
associated with the acoustic phonon sensitive to the shear elastic modulus
$\left(C_{11}-C_{12}\right)/2$. On warming, the shear mode softens for momentum
transfers near close to half the Brillouin zone boundary, but recovers near the
zone centre. High resolution spin-echo measurements further illustrate a
temporal broadening with increased temperature over this intermediate range of
wavevectors, indicative of a reduction in phonon lifetime. A subtle shift in
phonon frequencies over the same range of momentum transfers is observed with
magnetic fields. We discuss this acoustic anomaly in context of coupling to
orbital and charge fluctuations.
