[@Abragam:book] to an excited $j_{eff}$=1 triplet state. The splitting of such degenerate ground state due to a local molecular field could provide the spin-orbit crystal field level with the correct energy scale to couple to the acoustic phonon. However, we emphasise that such a coupling in the framework of the theories described above would involve a true softening of the fluctuations near the Brillouin zone centre inconsistent with the phonon anomalies in [[$\textrm{Mg}\textrm{V}_2\textrm{O}_4$]{}]{}.

Another possibility is coupling between the acoustic phonons and low energy magnon excitations from the V$^{3+}$ S=1 sites. No magnon excitations were observed to cross the phonon branch in our experiments, however, it should be noted that the comparatively large momentum transfers optimised for phonon measurements are where magnetic form factors ensure reduced neutron scattering cross sections from magnons. But, such magnetic excitations have been reported to be highly dispersive [@Wheeler10], gapped [@Gleason2014], and located in reciprocal space away from the low-energy acoustic lattice fluctuations, making coupling in the context of the theories discussed above likely weak. While full magnon dispersion curves from neutron scattering are limited in [[$\textrm{Mg}\textrm{V}_2\textrm{O}_4$]{}]{}, neutron measurements in ZnV$_{2}$O$_{4}$ [@Lee04] observe a significant gap of $\sim$ 10 meV at the magnetic zone centre 