In plain words
That is, looking into every single matter and finding out the real state of it has become a kind of bad habit with us. We are going along some road, and a bad smell comes. Getting away from it and moving on quickly, that we do not do. Where is this stink from? Has something died in that bush? Is there something under some stone? In many such ways we set about investigating. Through this the distortions of the mind grow still more. Through sight, hearing and smell, our thoughts grow ever more distorted. So, as far as we can, we must make the effort to control sound to some degree, to check it and to keep it in hand.
When we close these six senses tightly in this way, there we hear the nāda-bindu-kalā. You can find it out right now. If we close our ears tightly just once, we keep hearing a sound inside, "Oṃ –". Even where there is only the tiniest opening, the sound of the Praṇava keeps coming through it as sound. When we shut these doors and windows and a strong wind blows, if there is a small hole in some door, a sound, "Oṃ", keeps coming from there. In the same way, this mansion called the body has doorways called the ten senses. Of these, the chief are these four. The ears, the nose, the eyes and the mouth. When we close these four, we hear the inward sound of nāda and bindu. Just beyond this sound lies the light of the Ātma. That is why they have praised God with such a hymn as "Nāda-bindu-kalātīta, namo namo." So the power in this ṣaṇmukha mudrā lies in making us hear the sound of nāda and bindu. When we concentrate on this sound, these outward senses turn inward.
They have said that restraining the mind is itself Mokṣa. The mind is what matters for Mukti. What kind of mind should it be? It should be a mind filled with good thoughts. Mind, citta, Manas: these three are only synonyms. That which is held by evil thoughts, bad feelings, evil brooding and evil hearing they called Manas. That which is made of sacred feelings, sacred hearing and sacred sights they called citta. When it is pure and spotless, they called it cloth.