The sounds of not listening properly

I bought Simon and Garfunkel's album Bookends when it came out in 1968. I nearly wore out the vinyl on my copy over the summer of '68-'69.

But it wasn't until I listened to it on my mp3 player this afternoon that I heard, sampled in the background of Save The Life of My Child, the opening vocal from their 1965 hit The Sounds of Silence: Hello darkness my old friend/ I've come to talk with you again. It's at 1.20-1.26, during that spaced-out instrumental break.

I know I'm a poor listener, but I can't believe it took me nearly forty years to notice it.