How Bhajans Should NOT be Sung

Mata Amirtanandamayi, "Darling children, to gain concentration in this age of materialism (Kali Yuga), bhajan is easier than meditation. By loud singing, other distracting sounds will be overcome and concentration will be achieved. Bhajan, concentration and meditation, this is the progression. In fact, constant remembrance of God is meditation. Children, bhajan sung with one-pointedness, will benefit the singer, the listener and also Mother Nature. Such songs will awaken the listeners minds in due course. If bhajan is sung without concentration, it is a waste of energy. Bhajan is a spiritual discipline aimed at concentrating the mind on one's Beloved Deity. Through that one-pointedness, one can merge in the Divine Being and experience the Bliss of one's True Self."

Sweetness of devotion
Amma says, it matters not whether one believes in Krishna or Christ, Mother Kali or Mother Mary; a formless God or even a flame, a mountain or an ideal such as world peace can be meditated upon while singing. By letting the mind expand in the sound of the divine chanting, each one can enjoy the peace born of one's inherent divinity. Children, the sweetness of devotion is unique. Only if medicine is applied to the wound after cleaning it with antiseptic, will it heal. In the same way. In the same way, first purify your mind with the antiseptic of devotion to God and then apply the medicine of knowledge.

Sing bhajans loudly at dusk

At dusk, the atmosphere is full of impure vibrations. This is the time when day and night meet and is the best time for sadhaks to meditate because good concentration can be attained. If sadhana is not done, more worldly thoughts rise up. That is why bhajan should be sung loudly at dusk. In this way, the atmosphere will also be purified. Children, at dusk sing bhajan while sitting in front of a burning oil lamp. The smoke produced by the wick burning in oil is a siddha aushadha (perfect medicine). We inhale the smoke and the atmosphere is also purified.

http://www.amma.org/teachings/spiritual-practices/bhajans.html