Silence is the absence of sound. The title, then, is a paradox.
It's important to understand the historical context of folk rock. The world was recovering from a devastating war, not just physically and economically but psychologically. How do you live in a society that is capable of genocide? What does it mean to be human when members of your species consider industrialized murder not only possible but a worthy goal? What can you accomplish when nuclear war may break out at any moment and destroy everything? How can you be human in the midst of so much inhumanity? Another parodox.
The events of the song take place in a dream. There's tension all through the song between opposites: the ideal and the real, the individual and society, light and dark, sound and silence. The singer addresses darkness itself as a friend and walks through a dark city. There's no loneliness so acute as when you know there are people all around you. Simon's depression certainly informed this image.
The "neon god they made" is the central image. Is it television? Is it religion? Is it war? Yes, it is all those things and more. Everyone hears the message. Everyone follows the cultural imperative. No one wants to hear an alternative, much less consider it. There is an infinitude of options that do not end in greed and death. There are none so deaf as those who will not hear, and no sound will penetrate that willful ignorance. The singer makes a sound, but to the multitude it is silence, drowned out by orthodoxy. His words fall into a well of other wise thoughts, ready to be drawn out by those who will heed them. No one in this vision will heed.
Is the message still relevant? More than ever, but few are willing to listen. Our instinct as individuals is to survive, but our instinct as cultures is to destroy. The final paradox.