Sound is a kind of material form (rūpa)

A tremendous sound can damage the ear bones and cause deafness. This shows that sound is a kind of material form (rūpa), possessing the impact force of material form; therefore there is tactile sense-object (sparśa-viṣaya), which contacts the material ear and thereby damages it. When the coarse aspect of sound contacts the ear faculty (śrotra-indriya), the ear consciousness first discerns the coarse sound, and the mind consciousness afterward discerns its subtle aspects. Often, the ear consciousness first senses the vibration striking the ear faculty—these are the five sense-objects—and only then can the mind consciousness know what the specific sound is, along with other subtle dharmas.