Here's the bit from the novel you need:
... a bird, a vision of red and yellow, flashed upwards with a witch-like cry; and this cry was echoed by another.
“Hi!” it said. “Wait a minute!” The undergrowth at the side of the scar was shaken and a multitude of raindrops fell pattering.
“Wait a minute,” the voice said. “I got caught up.”
The fair boy stopped and jerked his stockings with an automatic gesture that made the jungle seem for a moment like the Home Counties.
The voice spoke again.
“I can’t hardly move with all these creeper things.”
Don't worry too much about the significance of the "voice" in this first chapter. All it is Piggy: the voice becomes the "fat boy", when he appears out of the creepers, then eventually "Piggy". "The voice" is just Piggy's voice, before Ralph knows who Piggy is.
It's a narrative technique that Golding uses throughout the novel: narrating from one (or all) of the boys' points of view. We're seeing and hearing Piggy through Ralph's ears: first he's just a "voice", then he's the "fat boy", and then, as Ralph finds out his name, he's Piggy.
Hope it helps!