We have used a custom-built ultrasound velocimeter to carry out high-precision velocity measurements of reverse micelle solutions, made of ionic (AOT) and nonionic (C12E4) surfactants in oil, as a function of water concentration. We show that the observed velocity variation as a function of increasing water concentration differs from the characteristics of the surfactant polar headgroups. The complex profile of compressibility curves obtained from velocity and densimetric measurements can be accounted for by the relation existing between the surface polar headgroup of each surfactant and the number of interacting water molecules. At the highest water concentration, the compressibility parameters obtained are different from those reported for "bulk" water and reflect the peculiar properties of confined water.