And they sung as it were a new song before the throne, and before the four beasts, and the elders. They sing; that is to say, the heavenly inhabitants. The four living beings; viz. those of Revelation 4:9, where see an explanation of the positions occupied, and of the nature and signification of the "living beings and the elders."
The "new song," which can only be understood by the hundred and forty-four thousand, is (as explained by Revelation 4:4) a song of victory won by those who have been tried in the world and subjected to temptations.
And no man could learn that song but the hundred and forty and four thousand, which were redeemed from the earth; even they that had been purchased out of the earth (Revised Version). These only can know the song for the reason given above.
The joys of heaven and the song of victory are not for those who have succumbed to the world.