Here the words "joyful sound" refer to the sound of people shouting and blowing horns. These were common acts of worship during Israelite festivals. (See: [[rc://en/ta/man/jit/figs-explicit]])
The writer speaks of Yahweh acting favorably towards them as if Yahweh's face shone a light on them. Alternate translation: "knowing that you act favorably towards them" (See: [[rc://en/ta/man/jit/figs-metaphor]])