The relationship between the sound velocity and density in various oxide compounds at atmospheric pressure is relevant to problems of the earth's interior. Here, data on elastic constants of various compounds are collected and analyzed. It is shown that the bulk modulus-volume per ion pair relationship for oxide compounds differs in a remarkable degree from that found for alkali halides, fluorides, selenides, sulfides, and covalent compounds. It is shown that a change of volume has the same effect on the bulk modulus of oxide compounds, whether the volume change is produced by pressure, compositional variation, phase changes, temperature, or porosity. It thus appears that volume is the primary variable affecting the elastic moduli of oxide compounds, and all other variables affect the moduli only insofar as they affect the volume itself.