Sound here means vibration, meaning the nerve currents which conduct it; and knowledge, reaction. All the various meditations we have had so far, Patanjali calls Savitarka (meditation with question). Later on he gives us higher and higher Dhyānas. In these that are called "with question," we keep the duality of subject and object, which results from the mixture of word, meaning, and knowledge. There is first the external vibration, the word. This, carried inward by the sense currents, is the meaning. After that there comes a reactionary wave in the Chitta, which is knowledge, but the mixture of these three makes up what we call knowledge. In all the meditations up to this we get this mixture as objects of meditation. The next Samadhi is higher.