"What sort of things did you hear, popkin?" breathed Aunt Petunia, very white-faced and with tears in her eyes.
But Dudley seemed incapable of saying. He shuddered again and shook his large blonde head, and despite the sense of numb dread that had settled on Harry since the arrival of the first owl, he felt a certain curiosity. Dementors caused a person to relive the worst moments of their life.... What wouls spoiled, pampered, bullying Dudley have been forced to hear?
american version, chapter two, page 30
I searched the book 5 forums and couldn't find another topic on this, for which I was very surprised. If I missed it, though, forgive me mods!

But anyway, what do you think Dudley would have been forced to hear? What terrible things in his life would he have had to relive?? I think there's more to him than we think.