Music and sound are running themes throughout the novel. It's no mistake that music is tied to the strike and to Galt's value system. Meanwhile, sound, unappealing, dangerous, and even deadly, is linked to the looters. As with many things in the novel, different aspects of a broad category – in this case types of sound – are used to symbolically differentiate the strikers from the looters.
Music is especially important to Dagny's character. When we first meet her, she is listening to music on a train:
She sat listening to the music. It was a symphony of triumph. The notes flowed up, they spoke of rising, and they were the rising itself, they were the essence and the form of upward motion, they seemed to embody every human act and thought that had ascent as its motive. . . .Somewhere on the edge of her mind, under the music, she heard the sound of train wheels. (188.8.131.52)
We actually learn Dagny's thoughts about and love of music, particularly Richard Halley's music, before we even learn her name. The style of this passage is notable; the sentence length is unusually long for this book, and the style is unusually fluid – there are no breaks between phrases. This may be a way to mimic the sound of the music itself and to emphasize Dagny's love of it. Her love of Halley's music, particularly his mysterious Fifth Concerto, signals her value system and the way in which she is already aligned, unknowingly, with the strike.
It is significant that music here is also linked to the sound of machinery and with motion. (Check out "Motors and Motion" below for more on this.) Galt's philosophy is a total one that united things like love, work, and happiness. So it's no mistake that music, which represents Dagny's views and philosophy, is directly tied to the railroad and Dagny's love of her work. Halley's Fifth Concerto emerges as a recurring motif, or repeated symbol, throughout the book, most notably during the John Galt Line's opening and Dagny's introduction to Atlantis:
She heard the sound of the waterfall before she saw the fragile thread that fell in broken strips of glitter down the ledges. The sound came through some dim beat in her mind, some faint rhythm that seemed no louder than a struggling memory...but another sound seemed to grow clearer, rising, not in her mind, but from somewhere among the leaves...she knew that she was hearing [the Fifth Concerto] now, hearing it rise from the keyboard of a piano. (184.108.40.206)
Again, Halley's concerto is directly linked to the theme of rising and movement, which are cast in a positive light. It's also notable that the music is tied to sounds of nature and the natural world. This helps indicate the type of peaceful place that Atlantis is, a where industry, art, and nature are in harmony.
The other Halley piece that plays a major role for Dagny is his Fourth Concerto. This one is often described as "tortured," and it's the one Dagny recalls in moments of pain and doubt, as during her plane crash in Atlantis:
She heard a piece of music in her mind, one she seldom liked to recall; not Halley's Fifth Concerto, but his Fourth, the cry of a tortured struggle, with the chords of its theme breaking through, like a distant vision to be reached. (220.127.116.11)
At the other end of the spectrum are the negative sound motifs associated with the looters. An example is the mysterious Project X, which uses sound waves to create a deadly weapon.
The machine is named the Thompson Harmonizer after it is unveiled, and Dr. Ferris explains that "there are certain frequencies of sound vibration which no structure, organic or inorganic, can withstand." (18.104.22.168)
This is a great metaphor for the way the looters operate. They use "sound" – their speeches, dialogue, and ideas – to effectively destroy the country. We see the effects of this destruction in the urban decay and in the individuals who are beaten down by the looters. It's no accident that characters like Hank and Dagny are perpetually unable to understand the looters and the seemingly meaningless and contradictory sounds coming out of their mouths. The looters' words are as deadly as their sound-wave weapon. While the music and sounds of Atlantis convey hope, the sounds linked with the looters bring destruction and even death.