Perhaps referring to soprano voices.

Psalms 46—48 praise God in His greatness, particularly in His city, Jerusalem. These psalms reveal how God dwells in the church as His city, His habitation, to be enjoyed by the saints, that in Christ He may be King over all the earth.

Signifying the flowing of the Triune God in Christ through the Spirit as life to His people (Rev. 22:1 and note Rev. 22:1^2b and note Rev. 22:1^3c).

The king in Psa. 45 typifies Christ as the King; the city in this psalm, and often in the Bible as a whole, signifies a kingdom. As the King, Christ needs a city in which to rule and reign. The city of God is the enlarged, strengthened, and built-up church as the ruling center for God’s reign in His kingdom. The church as the house of God (Psa. 23:6; 26:8; 27:4; 36:8; 1 Tim. 3:15) must be enlarged to become the church as the city, the kingdom, of God (Rev. 5:9-10). Eventually, in God’s economy the house of God becomes the holy city, New Jerusalem, as God’s eternal habitation and the ruling center of His eternal kingdom (Rev. 21:2-3, 22; 22:3).

The city, which cannot be moved, is the unshakable kingdom (Heb. 12:28), which is Christ Himself with His enlargement, His increase (Dan. 2:34-35, 44, and note Dan. 2:35^3b, par. 2).

This word spoken by God to the raging nations (v. 6a; Psa. 2:1-2) is a prophecy indicating that when the church is enlarged, strengthened, and built up to be a city as God’s kingdom (cf. Matt. 16:18), God will have the ground to deal with the nations and possess the entire earth (cf. Dan. 2:34-35; Rev. 19:11-21).