I'm not positive about this, but what if you take white noise (or pink noise to keep the frequency distortion to a minimum) and then add your hidden message to the left and right channels? Split it so that every other frequency band is on the other channel (L, R, L, R, etc) to keep it roughly even, and make sure the noise is on both channels equally (centered). The noise would drown out the message generally, and the message would only "come out" when the center was subtracted.
