Sound velocities have been measured in polycrystalline samples of silver iodide at 122¿C to pressures of 6¿108 N/m2. The data cover three different polymorphs within their respective stability fields. The low-pressure phase (B4 structure has pronounced negative pressure derivatives of both velocities, probably due to defects. The intermediate pressure phase (B23 structure) stable from ~2 to ~3.5¿108 N/m2, has anomalously low velocities and elastic moduli for its density. The high-pressure phase (B1 structure) has anomalously high dVp/dP and dK/dP, also possibly due to defects. The velocity and bulk modulus shifts at these transitions, especially the B4-B23 transition, are in accordance with a previous prediction of anomalous velocity shifts to be expected at a transition with a negative dP/dT.