As for public prayers, there are two kinds: the one consists simply of speech, the other of song…And indeed, we know from experience that singing has great strength and power to move and to set on fire the hearts of men in order that they may call upon God and praise him with a more vehement and more ardent zeal. It is to be remembered always that this singing should not be light or frivolous, but that it ought to have weight and majesty…Now, what Augustine says is true, namely that no one can sing anything worthy of God that he has not received from him. Therefore, even after we have carefully searched everywhere, we shall not find better or more appropriate songs to this end than the Psalms of David, inspired by the Holy Spirit. And for this reason, when we sing them, we are assured that God puts the words in our mouth, as if he himself were singing through us to exalt his glory.
Thanks to Wes Bredenhof for reminding us of this gem.