The velocity of sound in a water-saturated sediment is controlled by the distribution of interstitial microbubbles (no greater than 60 &mgr;m in diameter) rather than simply by total gas volume. A continuously increasing gas volume fraction in the sediment is accompanied by a discontinuous decrease of compressional wave velocity, and the overall velocity reduction is only 11% at a gas volume fraction of 0.18. Neither of these observations agrees with the predictions of either emulsion theory or theory for a fluid-saturated porous solid when modified to describe gas-bearing sediments.