They are probably a remnant from the evolutionary process much like the vestigial pelvic bones are no longer functional for supporting the weight to the hind quarters. The sounds manatees and other marine mammals can hear are detected through transmissions received from the middle and inner ear, just like the sounds you would hear if you were swimming under the water. Most marine mammals are better adapted to this feat, as they can usually isolate sounds received by other dense bones in the body from the sounds they detect in insulated inner ear chambers. Studies on auditory evoked responses in manatees show that they have very good general hearing capabilities. Along with other things they have hind-molar tooth replacement that gives them a lifetime supply of teeth.
New teeth erupt at the back of the jaw and work forward.
The teeth are rapidly worn away by sand and gritty material in their vegetarian diet. Like an elephant, they are long-lived and may live to be from 50 to 80 years old. When it dies you can count the growth-layers found in a small bone located in their inner ear. However, there is no way, as of yet, to prove that this method of aging is accurate. The biggest I have seen was over 3600 lbs.