One's memory of Kṛṣṇa is revived by chanting the mahā-mantra, Hare Kṛṣṇa. By this practice of chanting and hearing the sound vibration of the Supreme Lord, one's ear, tongue and mind are engaged.
(Bhagavad-Gita-----8:8-----purport).
In all the Vedic mantras the word oṁ, called praṇava, is a transcendental sound vibration and is also Kṛṣṇa. And because in all the hymns of the four Vedas-Sāma, Yajur, Ṛg and Atharva-the praṇava, or oṁkāra, is very prominent, it is understood to be Kṛṣṇa.
(Bhagavad-Gita-----9:17-----purport).

There is no difference between the Lord and sound vibration coming from Him, even though He is not personally present. The best way of understanding is to accept such divine instruction, and Brahmā, the prime spiritual master of everyone, is the living example of this process of receiving transcendental knowledge.
(Srimad Bhagavatam-----2:9:8-----purport).

If one thinks that the holy name of the Supreme Personality of Godhead and the names and activities of the demigods are on the same level, or if one accepts the holy name of Viṣṇu to be a material sound vibration, that is also an offense.
(Srimad Bhagavatam-----3:15:25-----purport).

By chanting the Hare Kṛṣṇa mantra one immediately concentrates on the sound vibration and thinks of the lotus feet of the Lord, and very quickly one is elevated to the position of samādhi, or trance. If one goes on chanting the holy names of the Lord, which are not different from the Supreme Personality of Godhead, naturally his mind becomes absorbed in thought of the Lord.
(Srimad Bhagavatam-----4:8:44-----purport).
Thus one can gloriously become freed from all material contamination by the simple process of chanting Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare. Just as life in this material world has its beginning in material sound, similarly a spiritual life has its beginning in this spiritual sound vibration.
(Srimad Bhagavatam-----4:23:17-----purport).

Vedic evidence is called śabda-brahma. There are many things which are beyond the perception of our imperfect senses, yet the authoritative evidence of sound vibration is perfect. The Vedas are known as śabda-brahma because evidence taken from the Vedas constitutes the ultimate understanding. This is because śabda-brahma, or the Vedas, represents the Supreme Personality of Godhead. However, the real essence of śabda-brahma is the chanting of the Hare Kṛṣṇa mantra. By vibrating this transcendental sound, the meaning of everything, both material and spiritual, is revealed. This Hare Kṛṣṇa is nondifferent from the Personality of Godhead. The meaning of everything is received through the air through sound vibration. The vibration may be material or spiritual, but without sound vibration no one can understand the meaning of anything.
(Srimad Bhagavatam-----4:24:40-----purport).

The origin of creation is sound vibration, and if the sound vibration is clear and purified, perfect knowledge and perfect activities actually become manifest. This is enacted by the chanting of the mahā-mantra, Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare.
(Srimad Bhagavatam-----4:24:43-----purport).

There is no difference between the Hare Kṛṣṇa mahā-mantra and oṁkāra because both of them are sound representations of the Supreme Personality of Godhead. praṇavaḥ sarva-vedeṣu. In all Vedic literatures, the sound vibration oṁkāra is the beginning. Oṁ namo bhagavate vāsudevāya. The difference between chanting oṁkāra and chanting the Hare Kṛṣṇa mantra is that the Hare Kṛṣṇa mantra may be chanted without consideration of the place or the sitting arrangements recommended in Bhagavad-gītā (6.11)
(Srimad Bhagavatam-----7:15:31-----purport).
The vibration of Kṛṣṇa's flute is the origin of the Vedic hymns. Lord Brahmā, who is seated on a lotus flower, heard the sound vibration of Kṛṣṇa's flute and was thereby initiated by the Gāyatrī mantra.
(Sri Caitanya Caritamrta-----2:8:138-----purport).

A bona fide spiritual master chants the holy names—Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare—and the transcendental sound vibration enters into the ear of the disciple, and if a disciple follows in the footsteps of his spiritual master and chants the holy name with similar respect, he actually comes to worship the transcendental name. When the transcendental name is worshiped by the devotee, the name Himself spreads His glories within the heart of a devotee.
(Teachings of Lord Caitanya).

That the sound vibration of the Supreme Lord is identical with the Supreme Lord is a fact. This is confirmed in the Nārada-pañcarātra:
vyaktaṁ hi bhagavān eva
sākṣān-nārāyaṇaḥ svayam
aṣṭākṣara-svarūpena
mukheṣu parivartate
"When the transcendental sound vibration is practiced by a conditioned soul, the Supreme Lord is present on his tongue."
(Teachings of Lord Caitanya).

In the Padma Purāṇa also it is stated, "The chanting of the Hare Kṛṣṇa mantra is present only on the lips of a person who has for many births worshiped Vāsudeva." It is further said in the Padma Purāṇa, "There is no difference between the holy name of the Lord and the Lord Himself. As such, the holy name is as perfect as the Lord Himself in fullness, purity and eternity. The holy name is not a material sound vibration, nor has it any material contamination." The holy name cannot, therefore, be chanted offenselessly by one who has failed to purify his senses. In other words, materialistic senses cannot properly chant the holy names of the Hare Kṛṣṇa mahā-mantra. But by adopting this chanting process, one is given a chance to actually purify himself, so that he may very soon chant offenselessly.
(Nectar of Devotion).

In his book Vidagdha-mādhava, Śrī Rūpa Gosvāmī thus describes the vibration of Kṛṣṇa's flute: "The sound vibration created by the flute of Kṛṣṇa wonderfully stopped Lord Śiva from playing his ḍiṇḍima drum, and the same flute has caused great sages like the four Kumāras to become disturbed in their meditation. It has caused Lord Brahmā, who was sitting on the lotus flower for the creative function, to become astonished. And Anantadeva, who was calmly holding all the planets on His hood, was moving in this way and that due to the transcendental vibration from Kṛṣṇa's flute, which penetrated through the covering of this universe and reached to the spiritual sky."
(Nectar of Devotion).

Thus by chanting the holy name of the Lord one can directly associate with the Supreme Lord by sound vibration. As one practices chanting this sound vibration, one passes through three stages of development: the offensive stage, the clearing stage and the transcendental stage. In the offensive stage of chanting one may desire all kinds of material happiness, but in the second stage one becomes clear of all material contamination. When one is situated on the transcendental stage, one attains the most coveted position—the stage of loving God. Lord Caitanya taught that this is the highest stage of perfection for human beings.
(Sri Caitanya Caritamrta-----preface).