Laboratory measurements of sound velocities of high-pressure minerals provide crucial information on the composition and constitution of the deep mantle via comparisons with observed seismic velocities. Calcium silicate (CaSiO3) perovskite (CaPv) is a high-pressure phase that occurs at depths greater than about 560 kilometres in the mantle1 and in the subducting oceanic crust2. However, measurements of the sound velocity of CaPv under the pressure and temperature conditions that are present at such depths have not previously been performed, because this phase is unquenchable (that is, it cannot be physically recovered to room conditions) at atmospheric pressure and adequate samples for such measurements are unavailable. Here we report in situ X-ray diffraction and ultrasonic-interferometry sound-velocity measurements at pressures of up to 23 gigapascals and temperatures of up to 1,700 kelvin (similar to the conditions at the bottom of the mantle transition region) using sintered polycrystalline samples of cubic CaPv converted from bulk glass and a multianvil apparatus. We find that cubic CaPv has a shear modulus of 126 ± 1 gigapascals (uncertainty of one standard deviation), which is about 26 per cent lower than theoretical predictions3,4 (about 171 gigapascals). This value leads to substantially lower sound velocities of basaltic compositions than those predicted for the pressure and temperature conditions at depths between 660 and 770 kilometres. This suggests accumulation of basaltic crust in the uppermost lower mantle, which is consistent with the observation of low-seismic-velocity signatures below 660 kilometres5,6 and the discovery of CaPv in natural diamond of super-deep origin7. These results could contribute to our understanding of the existence and behaviour of subducted crust materials in the deep mantle.