Sri Chinmoy: When two things are struck together like this, they produce a sound. When I strike my hands together like this, I produce a physical sound. Sound is normally the result of some impact. When we want to produce any sound, two things must be brought together in some way. Then only can we produce a sound.
AUM, however, is called the soundless sound because we do not strike any object with any other object in order to produce it. Because it is unstruck, it is known as the soundless sound. In Sanskrit, this phenomena is called Anahata, which means literally ‘unstruck’. We can hear the sound in the inmost recesses of our heart, but we ourselves do not do anything to create it; it is done spontaneously. We only receive the AUM or hear it. The sound which exists in the inner world, unstruck, is called Anahata.