The hills melting like wax (part C) could refer to the wicked( part C’) as they are sometimes likened to mountains of brass.The wicked will melt while the righteous will be preserved.
A.(1) The LORD reigneth; let the earth rejoice; let the multitude of isles be glad thereof. (2) Clouds and darkness are round about him: righteousness and judgment are the habitation of his throne.
B. (3) A fire goeth before him, and burneth up his enemies round about. (4) His lightnings enlightened the world: the earth saw, and trembled.
C.(5) The hills melted like wax at the presence of the LORD, at the presence of the Lord of the whole earth. (6) The heavens declare his righteousness, and all the people see his glory.
D. (7) Confounded be all they that serve graven images, that boast themselves of idols: worship him, all ye gods.
E.(8) Zion heard, and was glad; and the daughters of Judah rejoiced because of thy judgments, O LORD.
D’.(9) For thou, LORD, art high above all the earth: thou art exalted far above all gods.
C’.(10) Ye that love the LORD, hate evil: he preserveth the souls of his saints; he delivereth them out of the hand of the wicked.
B’.(11) Light is sown for the righteous, and gladness for the upright in heart.
A’.(12) Rejoice in the LORD, ye righteous; and give thanks at the remembrance of his holiness.