We report on the influence of elastic waves on the optical response and light emission in simultaneously photonic and phononic resonant cavities. Elastic waves couple with light through the acousto-optic interaction. Concurrent control of both light and sound through simultaneously photonic–phononic, often called phoxonic, band-gap structures is intended to advance both our understanding as well as our ability to manipulate light with sound and vise versa. In particular, co-localization of light and sound in phoxonic cavities could trigger nonlinear absorption and emission processes and lead to enhanced acousto-optic effects. We review our recent work on sound-controlled optical response and light emission in phoxonic cavities and investigate the limits of validity of the photoelastic model that describes light–sound interaction to first-order approximation. Moreover we present some preliminary results on silicon nitride nanobeam phoxonic devices.