Charles Ellicott Commentary Revelation 14:3

"and they sing as it were a new song before the throne, and before the four living creatures and the elders: and no man could learn the song save the hundred and forty and four thousand, [even] they that had been purchased out of the earth." — Revelation 14:3 (ASV)

And they . . .—Translate, And they (i.e., the heavenly harpers) sing a new song (the words “as it were” should perhaps be omitted) before the throne, and before the living creatures, and the elders (i.e., in the presence of God Himself, and creation, and the Church), and no one was able to learn the song except the hundred and forty-four thousand who were purchased from the earth.

It is a heavenly song, and no spirit dulled by earthly desires can learn it. The Spirit of the Lord can open the dull ear to hear and to rejoice in the songs of God’s saints. Amid the world-noises of Babylon men can neither hear nor sing correctly the Lord’s song (Psalms 137:4); but the redeemed (the purchased from the earth) of the Lord can come with singing to Zion (Isaiah 51:11).