There are three ways of chanting the Mahamantra – chanting within the mind, chanting softly and chanting loudly. But unless we are very steady and fixed in devotional service, it is very difficult for us to chant within the mind or even to chant softly. We are bombarded with so many external noises as well as internal noise from our own mind so much that some times, the lips keeps chanting Krishna's names, but mind keeps wandering hither and thither, without listening to the names. In order to keep our own mind in control and to make it hear Krishna's names, it is important that we chant the Holy Names at least loud enough for ourselves to hear. While glorifying the gopis of Vrindavana for their loud chanting of Krishna's glories, Sri Uddhava says in Srimad Bhagavatam verse 10.47.63
vande nanda vraja strinaam
paada renum abhikshnashah
yaasaam hari kathodgitam
punaati bhuvana trayam
"I repeatedly offer my respects to the dust from the feet of the women of Nanda Maharaja's cowherd village. When these gopis loudly chant the glories of Sri Krishna, the vibration purifies the three worlds."
In the above verse the words 'hari kathodgitam' expands as 'hari katha udgitam' which means loud chanting of glories of Hari. Loud chanting of Krishna's glories not only helps us, but that vibration can purify all the three worlds. Once our beloved spiritual master H H Mahavishnu Goswami Maharaj, when observing the construction workers labouring hard all day, in building the temple in Sri Sri Radha Neel Madhav dham Rajkot, asked one of the devotees, "How to make these laborers chant 16 rounds?" The devotee didn't know what to respond and replied that he had never thought of that. Maharaj, with great consideration said, "We have to think about them. We have to make them do pious deeds. But how can we do it?" The devotee remained silent. Maharaj gazed at the workers again for a while and then said, "When we sing Harinam, they hear. This is punya karma. So whenever we do arati, kirtan, Bhagavatam class etc in the temple hall, we should keep the mic on so that all these people can also hear it as they do their work. When they hear, it is accumulating their pious deeds as the ears are always open. When they keep hearing Krishna's names all day, in the evening when they go home to rest, it will continue to ring in their ears and slowly they will start chanting 1 round and some day they will go upto 16 rounds." Maharaj is very compassionate and eager to distribute Harinam to all living entities irrespective of whether they are people visiting temple or who is involved in building the temple. Even when Dwarka temple was constructed in 2003, the sculptors there happily said that building Dwarka temple was a very special experience in their life as they could get true love as well as bhagavat bhakti apart from the wages. Such is the greatness of acaryas like Maharaj.
In Caitanya Caritamrta Antya Lila 3.67-76 we find the following conversation between Caitanya Mahaprabhu and Haridas Thakura.
Caitanya Mahaprabhu asked Sri Haridasa Thakura, "On this earth there are many living entities, some moving and some not moving. What will happen to the trees, plants, insects and other living entities? How will they be delivered from material bondage?"
Haridasa Thakura replied, "My dear Lord, the deliverance of all moving and non-moving living entities takes place only by Your mercy. You have already granted this mercy and delivered them. You have loudly chanted the Hare Krishna mantra, and everyone, moving or not moving, has benefited by hearing it. My Lord, the moving entities who have heard Your loud sankirtana have already been delivered from bondage to the material world, and after the non-moving living entities like trees hear it, there is an echo. Actually, however, it is not an echo; it is the kirtana of the non-moving living entities. All this, although inconceivable, is possible by Your mercy. When loud chanting of the Hare Krishna mantra is performed all over the world by those who follow in Your footsteps, all living entities, moving and non-moving, dance in ecstatic devotional love."
Once a German Professor, Prof. Pater Porsch asked His Divine Grace Srila Prabhupada, "Should this chanting be loud? Or can it also be half loud, whisper or silently, mentally? Does it play any difference? Does it make any difference?" Prabhupada replied, "If you chant loudly then others can hear. They also take benefit. Even the birds and beasts they will hear and be benefited. Therefore loud chanting is recommended. So that even the birds, beasts, trees, plants, they can hear."
So let us chant the holy names loudly for the welfare of our own selves and of others.