The following is from a homily by John Chrysostom on Psalm 42. The translator is Alex Poulos at The Poulos Blog.
For in the songs of the world there is harm, ruin, and much that would lead to danger. For all the licentiousness and lawlessness of these songs bring about divisions in the soul. But in the spiritual psalms, there is great gain, great benefit, great sanctification, and every tenant of philosophy may be found. By these words, the soul is cleansed, and the Holy Spirit is quick to be with the one who sings in this manner. For those who sing with understanding invoke the grace of the Spirit, which is why Paul says, “do not get drunk on wine, in which there is debauchery. Instead, be filled with the Spirit.” Following this phrase on fullness, we hear, “singing and psalming in your hearts to the Lord.” What does it mean to sing “in your hearts to the Lord?” It means to sing with understanding, so that your mouth may not merely speak the words while your mind perishes, entirely deceived and separated. Instead, the soul should heed the tongue.