First lyrics by Pete Seeger.
Text from Chapter 35 of Deathly Hollows.
Second lyrics from Sound of Silence by Simon and Garfunkel.

The Next Adventure

Where have all the flowers gone, long time passing?
Where have all the flowers gone, long time ago?
Where have all the flowers gone?
Gone to young girls, every one!
When will they ever learn, when will they ever learn?

Dumbledore tells Harry that it was his willing sacrifice that made all the difference here.

Where have all the young girls gone, long time passing?
Where have all the young girls gone, long time ago?
Where have all the young girls gone?
Gone to young men, every one!
When will they ever learn, when will they ever learn?

What Voldemort killed was not the real Harry, but rather, that part of his own soul that resided in Harry. For the first time since he was a baby, Harry’s soul is wholly his own.

Where have all the young men gone, long time passing?
Where have all the young men gone, long time ago?
Where have all the young men gone?
Gone to soldiers, every one!
When will they ever learn, when will they ever learn?

Harry asks where they are, and Dumbledore, in turn, asks Harry where he thinks they are. He looks around and thinks it resembles an empty, cleaner King’s Cross Station. Dumbledore bursts into laughter, and Harry is annoyed.
Dumbledore explains the moment when he finally possessed all the Hallows. He realizes that he wasn’t fit to be the Master of Death – he could only master the Elder Wand, but not the other two Hallows.
Only Harry is the true master of all three Hallows, because the real master of Death doesn’t run away from it.

And where have all the soldiers gone, long time passing?
Where have all the soldiers gone, a long time ago?
Where have all the soldiers gone?
Gone to graveyards, every one!
When will they ever learn, when will they ever learn?

Finally, Dumbledore tells Harry that he has a choice – he can either go back or stay.

And where have all the graveyards gone, long time passing?
Where have all the graveyards gone, long time ago?
Where have all the graveyards gone?
Gone to flowers, every one!
When will they ever learn, oh when will they ever learn?

Harry looked at the pleased looking Headmaster, then at the squalling thing nearby and thought. A few moments later he got up and went over to the thing and picked back up his burden before heading to the now visible train in the station. With Dumbledore calling out to him in the receding background.

Hello darkness, my old friend
I’ve come to talk with you again
Because a vision softly creeping
Left its seeds while I was sleeping
And the vision that was planted in my brain
Still remains within the sound of silence

In restless dreams, I walked alone
Narrow streets of cobblestone
‘Neath the halo of a street lamp
I turned my collar to the cold and damp
When my eyes were stabbed by the flash of a neon light
That split the night, and touched the sound of silence

The silence encompassed him and reminded him of the quiet solitude he had in his cupboard, with only the spiders to keep him company. And looked at the now sleeping soul nestled in his arms as he got on the train with so many others.
Heading into “The Next Great Adventure.” And one he hoped a future that was better than his past.