As the Titanic first went below the surface after sinking, many survivors reported to have heard the rumbling and the breaking apart of the ship as it descended. While the ship reached a speed of 30–37mph, it would of reached the sea floor in around about 5 to 10 minutes landing on a relatively soft surface pushing out much of the acoustic vibrations.
At a distance of 2 and a half miles between the sea floor and the survivors, most of the sound and vibrations would have been dispersed over that much of a wide area and vibrations that reached the surface, if any; it would be indistinguishable to humans. Sound doesn’t transfer well to air, and humans can’t detect it above or below water or feel it as vibration.
Besides, it was extremely noisy at the surface with the screaming of a 1500 people freezing in the water.
