What's literally happening in the quote below is that Goodman Brown finds some people gathered around a rock in the woods. But the scene is transformed by imagery:
At one extremity of an open space, hemmed in by the dark wall of the forest, arose a rock, bearing some rude, natural resemblance either to an altar or a pulpit, and surrounded by four blazing pines, their tops aflame, their stems untouched, like candles at an evening meeting. The mass of foliage, that had overgrown the summit of the rock, was all on fire, blazing high into the night, and fitfully illuminating the whole field. Each pendent twig and leafy festoon was in a blaze. As the red light arose and fell, a numerous congregation alternately shone forth, then disappeared in shadow, and again grew, as it were, out of the darkness, peopling the heart of the solitary woods at once.
By mingling religious imagery with his descriptions of nature, Hawthorne creates the sense that sinister worship is happening here. First, Hawthorne sets the scene for a perverse religious gathering by describing the rock as pulpit-like. And this pulpit is surrounded by burning trees, which are akin to the candles lit during Christian worship. In Christian traditions, lit candles symbolize Christ, the "light of the world." But since this story associates nature with evil, all of Hawthorne's evocative descriptions of light in this passage take on a different connotation.
Before he was known as Satan, the devil was known as "Lucifer," which literally means "carrying light." So while the lights of candles in church are associated with Christ, this story clearly wants readers to see burning nature—treetops, leaves, twigs—as signs of devil worship. The story's imagery, which evocatively describes the natural world as a sort of perverse church, creates a sinister scene in which Goodman Brown sees that evil isn't wholly distinct from the town and religion in which he grew up—evil is actually a sort of mirror image of virtue, just as nature is a strange approximation of church.
When Goodman Brown finally finds the demonic gathering in the heart of the forest, the story emphasizes the importance of the moment with heightened imagery:
He paused, in a lull of the tempest that had driven him onward, and heard the swell of what seemed a hymn, rolling solemnly from a distance, with the weight of many voices. He knew the tune; it was a familiar one in the choir of the village meeting-house. The verse died heavily away, and was lengthened by a chorus, not of human voices, but of all the sounds of the benighted wilderness, pealing in awful harmony together. Goodman Brown cried out; and his cry was lost to his own ear, by its unison with the cry of the desert.
Imagery is a literary device that appeals to the senses, and in this case Hawthorne is engaging sound. Rather than seeing what's happening in the woods, Goodman Brown initially only hears it. This means that he (and the reader) must come to a gradual understanding of what's going on.
At first, Goodman Brown hears a familiar hymn, suggesting that perhaps he's stumbled on a Puritan religious service, the kind that he knows intimately from home. It seems like there's a crowd of voices singing, implying that a whole community might be there worshiping. This stands in odd juxtaposition with the story's preceding passage, which described Goodman Brown as "demonic" and "frightful" and engaged in "horrid blasphemy." For a moment, it seems possible that rather than being corrupted in the forest, he might be saved.
But immediately, the sound changes—from a choir of human voices singing a religious hymn, it becomes simply the sounds of the wilderness (associated with evil and sin) creating an "awful harmony," implying that something disturbing is happening. When Goodman Brown shouts, he can't hear his own voice because it simply joins the sounds of the wilderness. To suggest that Goodman Brown's human voice is indistinct from the frightening sounds of the wilderness underscores the story's message that humanity is part of nature and therefore prone to sin.