  In the context of recent experimental observations of an unexpectedly large
thermal Hall conductivity, $\kappa_H$, in insulating $\mathrm{La_2CuO_4}$ (LCO)
and $\mathrm{SrTiO_3}$ (STO), we theoretically explore conditions under which
acoustic phonons can give rise to such a large $\kappa_H$. Both the intrinsic
and extrinsic contributions to $\kappa_H$ are large in proportion to the
dielectric constant, $\epsilon$, and the flexoelectric coupling, $F$. While the
intrinsic contribution is still orders of magnitude smaller than the observed
effect, an extrinsic contribution proportional to the phonon mean free path
appears likely to account for the observations, at least in STO. We predict a
larger intrinsic $\kappa_H$ in certain insulating perovskites.
