73. When a person is in a state of natural well-being, he sings the psalms with a full voice and prefers to pray out loud. But when he is energized by the Holy Spirit, with gladness and completely at peace he sings and prays in the heart alone. The first condition is accompanied by a delusory joy, the second by spiritual tears and, thereafter, by a delight that loves stillness. For the remembrance of God, keeping its fervor because the voice is restrained, enables the heart to have thoughts that bring tears and are peaceful. In this way, with tears we sow seeds of prayer in the earth of the heart, hoping to reap the harvest in joy (cf. Ps. 126:5). But when we are weighed down by deep despondency, we should for a while sing psalms out loud, raising our voice with joyful expectation until the thick mist is dissolved by the warmth of song.
St Diadochos of Photiki On Spiritual Knowledge and Discrimination One Hundred Texts, Philokalia [V1] 278