My mouth speaks wise words, and the meditation of my heart will be full of insight. The two parts of the sentence are not entirely parallel. It starts in your heart; then your thoughts are put into words. What matters to the author is the wisdom and insight. Wisdom he has not of his own. See what follows in Psalm 49:4, where he speaks about his own manner of listening. His wisdom must come from above. See how the author realizes this, for instance, in Psalm 49:15: the Lord rules over life and death. This is a central theme in this psalm. Psalm 78 is a good example of the wisdom which you can receive by considering the history of the Lord God with his people Israel, Psalm 78:1, Give ear, O my people, to my teaching; incline your ears to the words of my mouth!
This verse is in contrast with Psalm 49:20, with which this Psalm ends: Man in his pomp yet without understanding is like the beasts that perish.
3 My mouth shall speak wisdom; the meditation of my heart shall be understanding.