Blessed is the people - Happy is their condition. See the notes at Psalms 1:1.
That know the joyful sound - That hear that sound. DeWette explains this of the call to the festivals and offerings, Leviticus 23:24; Numbers 10:10; Psalms 27:6. That is, says he, those who honor and worship God. The Hebrew word - תרועה terû‛âh - means a loud noise; a tumult; especially, shouts of joy, or rejoicing, Job 8:21; 1 Samuel 4:5; the “shout of a king,” that is, the joyful acclamations with which a king is welcomed, Numbers 23:21; the shout of battle, Jeremiah 4:19; Jeremiah 49:2. Then it means the sound or clangor of trumpets, Leviticus 25:9; Numbers 29:1-6. The word is, therefore, especially applicable to the sounding of the trumpets which attended the celebration of the great; festivals among the Hebrews, and there can be little doubt that this is the reference here. The idea is, that they are blessed or happy who are the worshippers of Yahweh, the true God; who are summoned to his service; who are convened to the place of his worship.
They shall walk, O Lord, in the light of thy countenance - They shall live in thy favor, and enjoy thy smiles.