Like the other rulers in Wisdom, the Soundkeeper has lost sight of what her people really need. Without Rhyme and Reason there to keep an overall balance, she tries to control what her subjects should value. In her case, this means taking away all the sounds and protecting them from ungrateful listeners.
So, this lady curses her entire kingdom to complete silence. But wait a second, she definitely doesn't obey this silence herself: "'For someone who loves silence, she certainly talks a great deal,' thought Milo" (12.57). Although she's actually being a tyrant, she thinks she's doing the right thing.
But by freeing the sounds from the Soundkeeper's fort, Milo seems to have already taught the Soundkeeper an important lesson: "you can't improve sound by having only silence. The problem is to use each at the proper time" (13.17). You need to do things in moderation, and you can't have everything under your own control.
Have you noticed that as much as Milo is learning along his journey, he's also becoming quite the teacher, too?