Determine where a shift in tone occurs here in The Great Gatsby. What/who causes it? How do diction, syntax, and sound devices change after it?
The only completely stationary object in the room was an enormous couch on which two young women were buoyed up as though upon an anchored balloon. They were both in white and their dresses were rippling and fluttering as if they had just been blown back in after a short flight around the house. I must have stood for a few moments listening to the whip and snap of the curtains and the groan of a picture on the wall. Then there was a boom as Tom Buchanan shut the rear windows and the caught wind died out about the room and the curtains and rugs and the two young women ballooned slowly to the floor.
Tom is the killjoy in this passage from The Great Gatsby, which is appropriate since he will play the same role when he smacks Myrtle and when he puts a stop to the flirtation, as he thinks of it, of Daisy and Gatsby. He causes the shift in this passage.
The writer, or more accurately, the speaker, relates the shift, however. Overall, he accomplishes the shift by reversing the imagery. The other rhetorical elements you mention contribute to the overall effect of the imagery.
The white dresses are "fluttering" and "rippling" as if just returning from a "flight," and the women are "buoyed up" as if they are floating in midair by an "anchored balloon." The diction contributes to the visual imagery. But the "whip" and "snap" and "groan" (diction contributing to sound imagery) become a "boom" and the shift occurs. Instead of the light, airy imagery, and in addition to "boom," we get "shut," "caught," "died," and "slowly to the floor." This juxtaposition--placing opposites side by side--of imagery, contributed to by the diction, enhances the effect of the shift. The speaker reverses the imagery, and thereby reverses the tone.
Of course, the light, poignant picture of the women floating is an illusion. And like other illusion in The Great Gatsby, it is brought to an abrupt end.
I think you are right. The sentence prior to your bolded introduces change however because Nick reports a quiet moment of just whips, snaps, and a groan which is different than the carnival feeling going on in the sentences before it.
Tom's entrance causes this shift. It's almost as if Tom Buchanan figuratively puts to rest the fun feel the rest of the characters in the room might be experiencing. His presence brings a stop to the shifting winds and gives a solid and dull permanence to the state of being in the room.
The diction of the sentence you bolded uses language figuratively to highlight this shift with a couple of words: the 'boom' suggests great change, and with force. Both these words characterize Tom. The word 'caught' to describe wind is unnecessary, but purposeful as these ladies have been trapped and there is an intensity when Tom is around. The word 'ballooned' is used figuratively. Typically it is used as a noun, here we see it an action. If you imagine a balloon falling slowly to the ground, it seems to go almost reluctantly, it wants to stay high in the air. I think the same is true with these ladies...
In terms of syntax, I see a sentence with 3 independent clauses in it whereas before, the sentences varied in structure between complex and simple sentences. This one highlighted is a compound sentence.