Obsession
Obsession—captured in Klausner's all-consuming preoccupation with detecting sounds too high-pitched for the human ear to hear—is the story's predominant theme. Dahl illustrates the nature of obsession by showing Klausner becoming increasing devoted to using the sound machine. While the story begins with Klausner calmly tinkering with the machine's electrical components, we see him becoming more absorbed in his work after the first successful test, waking first thing in the morning to begin another test. Klausner grows so entranced by his findings that he phones the doctor, with little consideration for the social awkwardness of waking someone with a call, to insist he come straight over to confirm Klausner's findings. After the machine breaks, Klausner's obsession loses its outlet, and he immediately transfers his preoccupation from the machine to the tree he believes he has wounded. Ultimately, Klausner's obsessiveness clouds his reality to the degree where forgets his manners and civility entirely, ordering the doctor to apply iodine to the tree's axe gashes while holding the axe threateningly.
Genius vs. Madness
Another of the story's major themes is the question of whether Klausner is an obsessive genius inventor or a madman. Whenever Dahl shifts into the doctor's point of view, it is clear that the doctor is debating this question in his head. The doctor assesses Klausner's mental state by evaluating his behavior and the plausibility of the things Klausner says. At the end of the story, the doctor obeys Klausner's demand to apply iodine to the tree when he sees Klausner's hand gripping the axe; this suggests that the doctor believes it is possible that Klausner is psychotic. However, Dahl leaves the question unanswered. The information crucial to knowing whether Klausner's machine works or he is imagining what he hears is kept from the reader, as the tree branch smashes the machine and prevents the doctor from giving a third-party opinion.
Personification of Plants
The attribution of human qualities to plants is another major theme in "The Sound Machine." When Klausner first explains the purpose of his high-pitched sound–detection machine, he does not mention plants. However, the shrieking sound he hears when Mrs. Saunders snips rose stems sends Klausner down a new investigative path. However, he reassures himself that he was wrong to call the sound a shriek of pain. For his next test, he swings an axe at the base of a tree in the park; appropriate to the tree's size, the sound of pain the tree makes is lower and more resonant than that of the roses. When Klausner recreates the experiment, the force of the axe swing seems to dislodge one of the tree's branches. Having heard the pained sound the tree made, Klausner reacts by demanding the doctor treat the tree's wounds as he would treat those on a human patient. In this way, the story tracks Klausner's gradual acceptance of the concept that plants can experience pain in the way humans do. By wishing to fix the tree's wounds, Klausner demonstrates he has personified the tree.
Subjective vs. Objective Reality
Working in tandem with the theme of genius vs. madness is the theme of subjective vs. objective reality. In narrating the story through a limited-omniscient narrator, Dahl can shift between characters' perspectives to present the story's events from multiple subjective realities. Dahl promotes critical thinking on the reader's part by refusing to provide an objective account of the events. To arrive at a conclusion as to whether Klausner's experience of reality is the objective truth or a subjective extension of his imagination, the reader must assess Klausner's behavior through a compilation of the doctor's, Mrs. Saunders, and the narrator's points of view. Ultimately, the ambiguous nature of the story suggests that it is impossible to have an objective perspective of reality, as reality is only ever experienced by a multiplicity of subjective points of view.
