Of course, silence doesn't have a sound. But the poetic phrase "sound of silence" and the question "what is the sound of one hand clapping?" do have their uses. In the song "The Sound of Silence" (Simon & Garfunkel), the word sound is used metaphorically, meaning meaning or consequence. And the consequences of silence are often injustice and the death of creativity.
The question, "What is the sound of one hand clapping?" is a call to sit in silence and listen in silence. When we listen in silence, our minds and hearts open, and we see the world in a whole new way. Another Zen story has a student ask, "How do I enter the Way?" The teacher replies, "Can you hear the flow of the distant stream?" The student says, "Yes," and the teacher answers, "Listen from there." The student then says, "And if I had not been able to hear the stream?" Then the teacher replied, "I would have told you to listen from there."
In listening to silence, we become attentive to all that is. Many musicians believe that silence is alive, and that all sound arises from it. Instead of seeing silence as empty, they see it as pregnant, full of the possibilities of all the music in the world.