When the Lamb (Jesus) had taken the book to himself to reveal its contents, the four living creatures and the twenty-four elders fell before his feet and worshiped him (Revelation 5:8). The text says that every one of them had harps and golden censors in their hands, however, I don’t believe this means the four beasts and the twenty-four elders had both a censor and a harp in his hands. If that were true, it would be difficult to use either, because both require two hands to use.
Probably, the four beasts had the harps for praising the Lamb, because praising the One on the throne seems to be their main function (Revelation 4:8), while the twenty-four elders’ main function would be to exercise their duties as priests, because they are the captains of the twenty-four courses of priests. They would each have a golden censor in their hands, which were full of the prayers of the saints. One of a priest’s functions was to offer prayers to the Lord before the Altar of Incense (cf. Revelation 8:3-4).
Does anyone sing praises to God without feeling, without understanding, or without truth? Would God receive such praise? If one sings a song praise, it should be for that which he is thankful to God for, i.e. what the Lord has done for the one praising him, or for what God has given that one. Since the book that the Lamb (Jesus) chose to disclose concerns what he has done for mankind, it seems logical that the praise that the Lord would accept could only come from mankind. While a heavenly being might look on in awe at what Jesus had done for men, how could he praise God in truth, understanding and with great emotion, if it were not done for himself.
No doubt any heavenly being could recognize in Jesus’ offices, as Priest, Prophet and King, the great character of God, and, in that context, they could praise him for who he is. However, how could any heavenly being praise God in truth, understanding and with feeling over redemption, since he has no idea what it means to be so destitute and without God, as the sinner is able to do, and, then, without just cause, experience the grace of God, when he receives that one back into his Kingdom? To praise God without having the need for God to redeem such a one, would be like trying to understand the scroll in the hand of the Lamb, before Jesus revealed its contents. Therefore, the Song of Redemption couldn’t have been sung by the four beasts. Rather, it was sung by the twenty-four elders, priests and representatives of the those who were redeemed by the blood of the Lamb.
“And they sung a new song, saying, ‘You are worthy to take the book, and to open the seals thereof: for you were slain, and have redeemed us to God by your blood out of every kindred, and tongue, and people, and nation; And have made us unto our God kings and priests: and we shall reign on the earth.” (Revelation 5:9-10 – emphasis mine)
Christ was crucified to redeem mankind, not heavenly beings, to God. Jesus’ blood was shed for sinners, not righteous heavenly beings who never sinned. Moreover, it is those who have been redeemed out of mankind who have been made a kingdom of priests to God (or kings and priests), and only men will reign on earth. Such a thing was never promised to any heavenly being. Therefore, except for John who witnessed this scene, the twenty-four elders are the only ones in heaven who could possibly have sung this song.