Piya wakes from a doze to a loud booming sound. She makes her way to Fokir and asks in mime what the sound was, but he only points vaguely and continues to watch Garjontola. When he starts to hum, Piya again encourages him to sing. Piya thinks this night feels perfect and magical. She wonders what kind of a life Fokir has led—if he saw his wife's face before the wedding, and if he grew up one of many children. Finally, Piya returns to her bed and listens to the dolphins in the water. She thinks of how they use sound to map their surroundings and talk to each other, and thinks of it in contrast to how she interacts from Fokir. She thinks that human speech is worthless compared to how dolphins communicate.
The booming sound is presumably the tiger that Moyna and Kanai heard, which again illustrates just how close the dangerous parts of the natural world are in the Sundarbans. When Piya compares human speech to dolphin echolocation, it shows that she's learned that nonverbal communication is far more effective than actual words.