What is meant by the “ethereal reverberations” of the Om vibration moving through the body? Perhaps an example might be helpful here: clap your hands three times and then lay them flat on the thighs, palms turned upwards. Feel the strong sensations in the palms. Continue focusing on the vibrations as the sensations get more and more subtle. Notice how the mind can easily track the vibrations as they get less and less obvious.
The important point in this example is this: as you continue focusing on the vibration while it gets quieter and less noticeable, you may “get” or understand that the vibration never really stops…and that the mind is capable of focusing indefinitely upon the subtle reverberations for as long as you choose to do so.
At a certain point, if you continue focusing upon the subtle vibrations (started in the palms), the mind—because it is focusing upon subtlety—begins to relax and then you may experience a visceral connection with the underlying “Hum” or “Om” of the body…like ethereal reverberations!