Psalm 33:1-3 Rejoice in the Lord, O ye righteous: for praise is comely for the upright. Praise the Lord with harp: sing unto him with the psaltery and an instrument of ten strings. Sing unto him a new song; play skillfully with a loud noise.
Rejoicing in the Lord includes praising Him with singing and with musical instruments. David responds to God with worshipful psalms and songs using a lyre or harp. Today we might use an organ, a piano, or a guitar, but the expression is still the same: rejoice, praise, sing. Beautiful music is an appropriate way to rejoice and praise the Lord.
Since God is always good and His mercy endures, it is fitting to sing and praise the Lord when believers gather and also to make melody in our heart to the Lord all during the day. This is one way we rejoice in the Lord. We also rejoice with shouts, prayer, thanksgiving, hearing the gospel, and by doing good and kind deeds to others. Much joy in the Lord is experienced when we live uprightly by faith. The abiding joy in the Lord finds expression in our feelings, our face, our words, and our interactions. The joy and peace of believing enters our hearts and comes out through our lips.