For God is King of all the earth The Psalmist, having called God in the close of the preceding verse King of the chosen people, now calls him King of all the earth; and thus, while he claims to the Jews the right and honor of primogeniture, he at the same time joins to them the Gentiles as associates and partakers with them of the same blessing. By these words he intimates that the kingdom of God would be much more magnificent and glorious at the coming of the Messiah, than it was under the shadowy dispensation of the Law, inasmuch as it would be extended to the utmost boundaries of the earth. To show the greater earnestness in his exhortation, he repeats the words, Sing praises to God, five times. The word mmkyl, maskil, [1] is put in the singular number instead of the plural; for he invites to this exercise all who are skillful in singing. He, no doubt, speaks of knowledge in the art of music; but he requires, at the same time, the worshippers of God to sing the praises of God intelligently, that there may not be the mere sound of tongues, as we know to be the case among the Papists. Knowledge of what is sung is required in order to engage in a proper manner in the singing of psalms, that the name of God may not be profaned, as it would certainly be, were there nothing more but the voice which melts away or is dissolved in the air.

Footnotes

1 - Calvin renders this word in the Latin version by "intelligens;" and in the French by "entendu;" and in the margin of the French version there is the note, "C'est, O vous chacun entundu!" -- "That is, O every one of you who understandeth!" Dr Adam Clarke reads, "Sing an instructive song;" and observes, "Let sense and sound go together. Let your hearts and heads go with your voices."

For God is the King of all the earth - He has shown himself to be a universal sovereign. All nations are subject to him, and he has a claim to universal praise.
Sing ye praises with understanding - Margin, Every one "that hath understanding." Neither the text here, however, nor the margin, expresses the true idea of the original. The Hebrew is, "Sing a Maschil" - משׂכיל maśkı̂yl; that is, Sing, or play, a didactic psalm or tune; that is, a song or ode adapted to convey valuable lessons of instruction. See the word explained in the notes at the title to Psalm 32:1-11. The idea is, that the occasion was one on which "such" a psalm or song would be especially appropriate; an occasion on which great lessons or truths had been taught by the dealings of God, which it became his people now to set forth in a becoming manner. Those lessons or truths pertained to the fact that God is the great King over all the earth, or that he is a sovereign among the nations: a truth of immense importance to mankind, and a truth which the occasion on which the psalm was composed was especially adapted to bring to view.

For God is the King of all the earth - He is not your King only, but the King of the universe. He has no limited power, no confined dominion.
Sing ye praises with understanding - זמרו משכיל zammeru maskil, sing an instructive song. Let sense and sound go together. Let your hearts and heads go with your voices. Understand what you sing; and feel what you understand; and let the song be what will give instruction in righteousness to them that hear it. Sing wisely - Anglo-Saxon. Multitudes sing foolishly.

For God [is] the King of all the earth: sing ye praises with (e) understanding.
(e) He requires that understanding be joined with singing, lest the Name of God be profaned with vain crying.

For God is the King of all the earth,.... Or "the king of all the earth is God" (e); the same that is ascended into heaven, and is King of saints, even Christ Jesus; and so he will appear to be, especially in the latter day; See Gill on Psalm 47:2;
sing ye praises with understanding; or, as De Dieu renders it, to him that understandeth, that is, to God the only wise, whose understanding is infinite; even to Christ, who, as God, knows all things; and, as man and Mediator, is of quick understanding, and has all the treasures of wisdom and knowledge in him; so R. Obadiah, "sing of him who understands"; or, "sing ye praises, O everyone that understandeth" (f); that is, how to sing, as everyone does not; this is the sense of Aben Ezra and Kimchi; or "with understanding", as we render it; with understanding of what is sung. The Apostle Paul seems to refer to this passage in 1-Corinthians 14:15. The Targum renders it, "with a good understanding".
(e) So Junius & Tremellius, Piscator, Cocceius, Michaelis. (f) "canite, intelligens", Montanus; i.e. "unusquisque", Vatablus.

The king - Not only ours, but of all the nations of the world. Sing - Not formally and carelessly, but seriously, considering the greatness of this king whom you praise, and what abundant cause you have to praise him.