"On August 29th, 1968, all the televisions in America shut down. There was a murmuring on the TV that some believe was the devil's voice. The televisions were off for about 25 seconds. No one knows what the issue was an no one knows what the sound was from the TVs."
I cannot corroborate that any such event--an inarguably remarkable one--occurred, which is surprising. There were significant power outages in both 1965 and 1968 that blacked out large portions of the country. UFO conspiricists love to mull over those. And it does remind me of the Doctor Who episode "The Impossible Astronaut" about and alien species known as The Silence that is set in 1969.
However, if the meme is real, it is interesting to note that this would have transpired one day after a famous antiwar protest at a Democratic National Convention in Chicago. The same antiwar demonstration that gave rise to the popular protest slogan: "The Whole World Is Watching."