Albert Barnes Commentary Psalms 89:15

"Blessed is the people that know the joyful sound: They walk, O Jehovah, in the light of thy countenance." — Psalms 89:15 (ASV)

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 as referring to the call to the festivals and offerings (Leviticus 23:24; Numbers 10:10; Psalms 27:6). That is, he says, 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); or the shout of battle (Jeremiah 4:19; Jeremiah 49:2). It also 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 your countenance - They shall live in your favor and enjoy your smiles.