Yea, they shall sing in the ways of the Lord - In the ways which God has appointed. They shall join with all that love him - with the humblest of the people - in acknowledging God. Kings and people shall thus bow before God in common acts of praise, and as being on the same level before him. As people, as sinners, as redeemed, as traveling to the grave, they are all alike before God.
For great is the glory of the Lord - Great is his character; great his dignity; great his honor; and all this will be seen to be so when those of most exalted rank thus worship and adore him. The most lofty on earth shall acknowledge that there is one who is more exalted than they are, and their own dignity and splendor shall thus contribute to deepen the impression of the honor and glory of God.

They shall sing in the ways of the Lord - They shall admire thy conduct, and the wondrous workings of thy providence, if they should not even unite with thy people.

Yea, they shall sing in the ways of the Lord,.... Which are all mercy and truth; ways of pleasantness, and paths of peace: so the eunuch went on his way, and in the ways of the Lord rejoicing, Acts 8:39; Or, "they shall sing of the ways of the Lord" (t); of the excellency, pleasure, and usefulness of them;
for great is the glory of the Lord; shown in the works of creation; more especially in the person of Christ, and in the glorious work of redemption and salvation by him; and of which there will be a great display throughout the earth in the latter day, by means of the Gospel, the great spread of it, and the multitude of persons converted by it; which will make the ways of the Lord still more pleasant; see Isaiah 6:3.
(t) "de viis Jehovae", Piscator, Schmidt; so some in Vatablus.

for great is the glory--or, "when the glory shall be great," in God's fulfilling His purposes of redemption.

The ways - His wonderful counsel and gracious providences.