We have measured the velocity of sound in a sandstone saturated with oil and brine at different concentrations. The velocity varations with the concentration depend drastically on the way of entering the fluids in the sample :injection of non-wetting fluid in a sandstone fully saturated with a totally wetting fluid (drainage) or vice versa (imbibition). We interpret our measurements with an extension of the Biot-Gassmann theory to a wetted frame of the sandstone saturated with an effective oil-brine fluid.