In fact, in Caitanya Bhagavata itself it has been described that Haridasa Thakura was loudly chanting the maha-mantra and an arrogant brahmana forbade him to do so and it was then that Haridasa Thakura quoted the verse that loud chanting is 100 times more powerful than silent chanting. There are many other instances and verses which can be quoted. Someday I want write a paper with all the proof against this statement that the maha-mantra should not be loudly chanted. I personally feel that silent chanting is for very very advanced devotees and that also they do it sometimes. I have seen the devotees who do not move even their lips while chanting. Many of them appear very morose throughout the day, don't look very inspired, get deviated easily, associate with the opposite sex, etc. This is just an observation.
Comment: In Vrindavan one time, I mentioned to Dr. O.B.L. Kapoor that this was not a bona fide mantra and he answered that it was not a mantra, it was a song that instructed us to worship Radha Govinda and Gaura Nitai, and to chant japa of the maha mantra.
A: I thought Bhaktisiddhanta said that bhaja nitai-gaura radhe shyam is a rasabhasa because they are including Nityananda Prabhu's name in the same line as Radhe Shyam. Bhaktisiddhanta did not say that it was song but considered it a man-made, chora nama:
"Sarasvati Thakura also told his disciples: "You should make a written notice and put it on every tree in every village, that this Nitai Gaura Radhe Syam is not a proper mantra, and is apasiddhantic. This is man-made, chora nama (not mentioned by any acarya). The real mantra is Hare Krsna Hare Krsna..."
"Another chant popular in Bengal and Orissa was "Sri Krsna Caitanya Prabhu Nityananda, Hare Krsna Hare Rama, Sri Radhe Govinda," which was not objected to by Sarasvati Thakura, though he never chanted it himself. He never said that this was rasa bhasa, saying that Nitai Gaura Radhe Syam was rasa bhasa, as they put Nitai Gaura and Radhe Syam in one line together, whereas the Hare Krsna Hare Rama, Sri Radhe Govinda is on a separate line."
(from Bhakti Vikasa Swami's notes on Bhaktisiddhanta Sarasvati)