Thought itself is a subtle form of speech. We can observe this for ourselves. Our thoughts are a kind of internal conversation, our own mental talk, often framed as a soliloquy or an imagined dialogue. If we replace this mental speech with mantra, we can transform the energy of our minds and hearts. This higher speech of the mind is the basis of buddhi or higher intelligence in Yoga, which allows our thoughts and actions to flow with the Divine word. Notice that you cannot see your mind or your thoughts, which have no visible form. The mind consists of sound which carries our memories, emotions and ideas. It is the power of sound that holds who we are, who we have been and who we wish to be. This is not just sound as one of the five elements but the primal sound vibration behind all the elements. Mantra as primal sound can change the nature of the mind down to the subconscious level. Through mantra, the control of both the mind and the senses becomes possible, not through the ego but through the Divine presence within us.

Frawley, David. Mantra Yoga and the Primal Sound (p. 48). Lotus Press. Kindle Edition.