Upon entering the Valley of Sound, Milo can immediately tell why Dischord and Dynne seemed to be so afraid of it: there is no sound whatsoever. As Tock frets over his lack of ticking, a crowd of people bearing protest signs about the silence comes down the road. One of them produces a blackboard and writes out the story of the Valley's loss. He explains that the Soundkeeper, once the benevolent ruler of the Valley, became upset with the lack of appreciation for beautiful sounds and the rise of Dr. Dischord's noises, and she, consequently, locked away all the sounds in her fortress. The blackboard man asks Milo to help by visiting the Soundkeeper and stealing a sound that they can use to destroy the fortress. Milo agrees and, with Tock and the Humbug sets out for the fortress.
The Soundkeeper turns out to be a perfectly pleasant woman who invites the travelers inside where they find sounds still exist. She tours them through the vault where she keeps every sound that was every made, showing as an example the "exact tune that George Washington whistled when he crossed the Delaware on that icy night in 1777." The Soundkeeper demonstrates how she catalogues sound by having Milo speak then walking him over to a filing cabinet where, sure enough, his "hello" is neatly stored.
Next on the tour is the laboratory where sounds are invented—and also become visible. Milo claps his hands and sheets of paper begin shooting out. He tries to reason with the Soundkeeper about the silence in the valley, but she will not hear of it. At one point Milo tries to retort with "But!" and catches himself short, feeling the word form on his tongue and stay there. He quickly, and silently, makes for the door, smuggling his sound out with him.
Milo hurries back to the angry mob where he is directed to deposit his stolen sound into a large cannon. When fired, the sound shatters the walls of the fortress and all the sounds in the vault come crashing out. After a few moments of noisy confusion, the dust settles, and Milo sees the Soundkeeper sitting on a pile of rubble. He goes over to apologize for helping destroy her fortress, and the Soundkeeper is very understanding. She recognizes that it was her own fault for taking away all the sounds and realizes that silence is not the solution. "The problem," she says, "is to use each sound at the proper time."
Dynne comes running up with an enormous sack full of sounds that are not noisy enough for his tastes. It turns out that he has gathered up nearly all the sounds of the vault, which the Soundkeeper happily accepts. As Dynne hurries away, the Soundkeeper remarks that the unpleasant sounds he likes have value because they help us appreciate the pleasant ones more. If only Rhyme and Reason would return, she says, they might be able to bridge that gap. She gives Milo a package of sounds to remember her by and bids him good luck with his quest.
Milo, Tock, and the Humbug pile back in the car and soon find themselves driving along the Sea of Knowledge. When each of the three makes an unfounded assumption about the ease of their quest, they leap out of the car and find themselves standing on the island of Conclusions. Unfortunately, they discover that it's much easier to jump to conclusions than to jump away; they have to swim back to shore. On the other side of the shore, Milo and Tock are soaked through with the waters of Knowledge while the Humbug, who thinks he already knows everything, is bone dry. Milo comments that he has learned a lesson about jumping to conclusions, and the three travelers are soon on their way again.
In the Valley of Sound, Juster ties Milo's lesson on perspective from Dischord and Dynne to the larger theme of appreciating everyday life. Just as Milo fails to appreciate the beauty of noises, the doctor and his sidekick are unable to appreciate the nicer sounds. When he reaches the Valley of Sound, Milo learns what happens when too many people take Dr. Dischord's perspective. Just like Milo and Dischord, the residents of this valley have an unbalanced appreciation of sounds—they seem to prefer one kind to the extreme. As Milo learns, the residents of the valley became so unable to appreciate beautiful sounds that the Soundkeeper takes them away along with all the noises as well. Notice the plot similarity here with the city of Reality, whose city disappeared because they failed to appreciate it. It seems that what people take for granted in the Lands Beyond disappears.
At the Soundkeeper's fortress, Milo learns about sounds in much the same way he learned about words in Dictionopolis. Sounds become tangible just like words at the Royal Banquet or in the word market. When he is actually able to see sounds in their "true" forms and the elaborate catalogue system that the Soundkeeper uses to keep track of them all, Milo seems to gain a greater appreciation of both noises and nice sounds. When Milo catches the word "but" in his mouth, Juster returns to his motif of punning by playing off the popular expression "on the tip of his tongue." Once the sounds have been restored, the Soundkeeper realizes that her inability to appreciate all sounds makes her just as bad as Dischord and Dynne. Beauty, as we learned in the last section, is a matter of perspective. The Soundkeeper realizes the sense of this, referring to Rhyme and Reason, symbols of wisdom, and declares that she will try to use each sound at the right time.
Milo learns a second lesson in this section during his detour to the island of Conclusions. Once they pass through the Valley of Sound, Milo and his companions each makes a statement that demonstrates an assumption. They make decisions without having enough information to do so and end up literally jumping to Conclusions. Since he got to Conclusions by deciding something before having enough information, it is fitting that Milo must swim through the Sea of Knowledge to get back to shore. When he finally makes it to the beach, Milo finds himself drenched in the waters of knowledge while the Humbug, who seems to be more interested in sounding knowledgeable than actually being knowledgeable, is bone dry. Juster seems to be telling us that one has to be open to knowledge in order to absorb it.