Voice and song, from deep feeling of heart

Calvin's Institutes (1559)
Book 3 of 4 - How We Receive the Grace of Christ

Chapter 20 - Prayer as exercise of faith and way we receive God's benefits

Sections 31-33 - Prayer in song, and the common language

31. Singing must come from the heart - Isa 29:13. It helps keep the mind attentive.

32. Church singing is legitimate, from 1 Cor 14:15; Col 3:16, and practiced in the early church. "Such songs as have been composed only for sweetness and delight of the ear are unbecoming to the majesty of the church and cannot but displease God in the highest degree."

33. Public prayer must be in the language of the people - 1 Cor 14:15-17. Private prayer need not be spoken, though when aroused this will often happen - 1 Sam 1:13.