And a voice came out of the throne, saying, Praise our God, all ye his servants, and ye that fear him, both small and great. (Rev. 19:5 KJV)
There are several views on the voice that came out of the throne. One possibility is it’s the voice of the glorified Christ speaking from his humanity. This would make sense because he uses the words “our God”. There is a time when the need of the son-ship (mediator) will end.
24 Then cometh the end, when he shall have delivered up the kingdom to God, even the Father; when he shall have put down all rule and all authority and power.
25 For he must reign, till he hath put all enemies under his feet.
26 The last enemy that shall be destroyed is death.
27 For he hath put all things under his feet. But when he saith all things are put under him, it is manifest that he is excepted, which did put all things under him.
28 And when all things shall be subdued unto him, then shall the Son also himself be subject unto him that put all things under him, that God may be all in all. (1 Cor. 15:24-28 KJV)
Even though the role of the sonship will no longer be needed, the son will not cease to exist. We see in 1 Cor 15:28 that the son will continue to exist and be subject to God.
Another possibility is the saints are speaking from the throne. Jesus says that he will grant us to sit on the throne with him.
To him that overcometh will I grant to sit with me in my throne, even as I also overcame, and am set down with my Father in his throne. (Rev. 3:21 KJV)
Everyone is to praise God no matter how great or small they are; no matter their rank.