8 There have not been wanting those who preferred understanding all the preceding passage also of the Prophet's own person; and would have even this verse, “Mine heart has uttered forth a good word,” understood as spoken by the Prophet, supposed to be uttering a hymn. For whoever utters a hymn to God, his heart is, as it were, “uttering forth a good word,” just as his heart who blasphemes God, is uttering forth an evil word. So that even by what follows, “I speak of the things which I have made unto the King,” he meant to express that man's chief work was but to praise God. To Him it belongs to satisfy you, by His beauty; to you to praise Him with thanksgiving....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)