"...The Words of the prophets are written on the subway walls and tenement halls.."
I believe these lyrics, and the rest of the song hinge on the Old Testament in the book of Daniel, 5:1-31.
At the banquet of King Belshazzar of Babylonia, the last pagan Babylonian king to rule over the Jews, all are feasting on golden plates and goblets, the sacred Jewish vessels, when a ghostly hand appears and begins to write on the wall. The King summoned all of his conjurers and diviners, to whom he offered great riches if they could decipher the writings. None could. So Daniel, a lonely Jew exiled in Babylon, was sent for. He had interpreted other signs and portents and it was hoped that he could interpreted the writings on the wall.
Daniel cursed the King for his cruelty, his greed, his thoughtlessness, his selfishness and his unkindness. He interprets the prophetic message: Belshazzar's kingdom will be destroyed and it will be given to the Persians and Medes.
Belshazzar makes good his word and makes Daniel 3rd in power in the kingdom, an honor Daniel does not wish. Later that night Belshazzar is killed and his kingdom is captured by the Persians and Medes.
The "writing on the wall" has become slang for events have been foretold, things haver been coming for some time with no one paying attention, bad things are about to happen.
I think Paul Simon personally witnessed every day the inequities of our society where so few have so much and so many have so little. He saw sick people suffer when there existed resources to help everyone. No one wanting war and yet violence existing everywhere.
I think in the song, Paul Simon becomes "Daniel", the lonely Jew, and Paul curses our society for it's cruelty, it's greed, it's thoughtlessness, It's selfishness and it's unkindness. A society that, despite it's frantic claims, is also a soulless, godless, "pagan" one, whose "Neon God," is one of commercialism, self aggrandizement, greed and worship of the dollar bill.
It's a society of hypocrisy and lies, so strong and loud that it deafens rational thought, quiets compassionate action, silences truthful words.
Why the "words of the prophets" warning of destruction "are written on the Subway walls, and tenement halls," is, I believe, because these words are not mysterious, not cryptic, need no interpreter or conjurers to divine them, they are so clear, so self-evident that even the most simple, the most common people can understand them. And they have always, in their hearts, knew that that the madness of our self-created society couldn't last forever. It is The People, in fact, who are the ones that have written the words on their own walls, not some God or supernatural presence.
Paul's prophetic dream has, I believe, come true with epidemics of addictions of all kinds, meaningless sex, nomadic marriages, the emptiness of hypocritical religions, endless war, guns and gun violence, abuse of The Weak, new diseases, faithless rulers, a race for escape to fantasy worlds of movie, TV, video games, anything to relieve boredom or gain a moment's instant gratification.
I see even more reason to fear "the sounds of silence" today with a true demagogue as our "leader," a man who can not breath without lying. His callousness, his cruelty, his greed, his thoughtlessness, his selfishness and his unkindness his lifelong to pleasure himself at ever turn, no matter what the cost to anyone else, makes me fear as a nation and society, but also all of the other nation's of the worlds also.
Just my interpretation. -Pete