There may be a lot of questions when we start chanting Hare Krishna; and one of them that frequently comes up is, "How loud can we chant?" or "can we chant in the mind?" Srila Prabhupada answers an inquisitive student in this conversation excerpt.
English guest: Your Grace? Must your chanting always be audible? I ask this, because I was chanting in the train. The train makes a noise. Now, I am surrounded by people.
Prabhupada: No?
English guest: I can chant audibly?
Prabhupada: Audible . . . if you chant even silently, it is also audible. You have to turn your attention, is it not?
English guest: Yes.
Prabhupada: Audible, if you don't make any sound, still it is audible.
Syamasundara: We hear it.
English guest: Would it be a good thing to chant audibly on a train full of people?
Prabhupada: Yes. Why not? They will also hear. They will be benefited. Yes. Haridasa Thakura used to do that. What to speak of human being, even animals, the insect, trees, if they hear, they will be benefited. It is so nice.
(aside as guests enter) Aiye. Accha. Jaya.
Whoever hears, he will be benefited. Just like sunshine—anyone comes in the sunshine, he will be benefited.