Rev 19:1 And after these things I heard a great voice of much people in heaven, saying, Alleluia; Salvation, and glory, and honour, and power, unto the Lord our God:
A great voice, people in heaven together before the coming of Christ to the earth saying, Alleluia; Salvation, and glory, and honour, and power, unto the Lord our God:
Rev 19:2 For true and righteous are his judgments: for he hath judged the great whore, which did corrupt the earth with her fornication, and hath avenged the blood of his servants at her hand.
These were the saints who came out of the great tribulation.
Rev 7:9 After this I beheld, and, lo, a great multitude, which no man could number, of all nations, and kindreds, and people, and tongues, stood before the throne, and before the Lamb, clothed with white robes, and palms in their hands;
Rev 7:10 And cried with a loud voice, saying, Salvation to our God which sitteth upon the throne, and unto the Lamb.
Rev 7:13 And one of the elders answered, saying unto me, What are these which are arrayed in white robes? and whence came they?
Rev 7:14 And I said unto him, Sir, thou knowest. And he said to me, These are they which came out of great tribulation, and have washed their robes, and made them white in the blood of the Lamb.
This was a time of celebration for the people who suffered in the world under the antichrist and were martyred. Perhaps the raptured church was here too, perhaps some were included in Rev 6.
Rev 6:9 And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held:
Rev 6:10 And they cried with a loud voice, saying, How long, O Lord, holy and true, dost thou not judge and avenge our blood on them that dwell on the earth?
The martyrs speak the highest word of praise: Aleluia (praise ye the LORD)
Rev 19:3 And again they said, Alleluia. And her smoke rose up for ever and ever.
In Revelation 18, Babylon's friends mourned her fall; but here, God's people celebrate it. The smoke of the evil that was the world system, shall receive everlasting torment. The fire shall burn forever. Is there any earthly pleasure worth eternal torment?
Rev 19:4 And the four and twenty elders and the four beasts fell down and worshipped God that sat on the throne, saying, Amen; Alleluia.
The twenty four elders and the four angels, or heavenly creatures that were in the throne room, fell down worshipping God who sat on the throne, saying, Amen; (truly) Aleluia.
Rev 19:5 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.
Perhaps the voice was Jesus. The world mourned the destruction of Babylon as the angels and saints shouted praises.