Charles Ellicott 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)

That know the joyful sound — that is, that are familiar with the shouting and music that accompanied the feasts of Israel.

They shall walk. — Better in the present tense; and so for the verb in the next verse. The light of Jehovah’s countenance, of course, means His favour.