Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Council in Trullo; Often Called The Quinisext Council.: Canon LXXV

Canon LXXV.

We will that those whose office it is to sing in the churches do not use undisciplined vociferations, nor force nature to shouting, nor adopt any of those modes which are incongruous and unsuitable for the church:  but that they offer the psalmody to God, who is the observer of secrets, with great attention and compunction.  For the Sacred Oracle taught that the Sons of Israel were to be pious. 382

Notes.

Ancient Epitome of Canon LXXV.

Inordinate vociferation of the psalms is not allowed, nor he that adopts things unsuited to the churches.

This question of the character of church-music was one early discussed among Christians, and (long before the time of this synod), St. Augustine, in debating as to whether the chanting or the reading of the psalter was the more edifying, concludes, “when the psalms are chanted with a voice and most suitable modulation (liquida voce et convenientissima modulatione), I recognize that there is great utility in the practice,” and further on he adds that singing is to be the rather approved, because “by the delight given to the ears the infirm soul is worked up to pious aspirations.”  (Confess. Lib. x., cap. xxxiij.).

Footnotes

The Latin adds, “and holy.”