How loudly should one chant?
How loudly you chant is a personal choice. Srila Prabhupada encouraged the devotees to loudly chant the Holy Name during kirtana, but when you're chanting japa together with others in a temple room or elsewhere, you should not interfere with other people's chanting by being too loud with your own rounds.
"Chanting a mantra or hymn softly and slowly is called japa, and chanting the same mantra loudly is called kirtana. For example, uttering the maha-mantra (Hare Krsna, Hare Krsna, Krsna Krsna, Hare Hare. Hare Rama, Hare Rama, Rama Rama, Hare Hare) very softly, only for one's own hearing, is called japa. Chanting the same mantra loudly for being heard by all others is called kirtana. The maha-mantra can be used for japa and kirtana also. When japa is practiced it is for the personal benefit of the chanter, but when kirtana is performed it is for the benefit of all others who may hear. In the Padma Purana there is a statement: "For any person who is chanting the holy name either softly or loudly, the paths to liberation and even heavenly happiness are at once open."
– Nectar of Devotion, Chapter 9