We had earlier been told about twenty-four elders on twenty-four thrones (Revelation 4:4), four living creatures (Revelation 4:6), myriads of angels (Revelation 5:11), as well as the souls of martyrs under the altar (Revelation 6:9) and multitudes of the elect before the throne (Revelation 7:9), and so we could come with reasons why we think one or more of these groups make the “loud voices.” But the fact that the Lord has not told us who generated the “loud voices” makes the answer to the question irrelevant to understanding the paragraph. The question of where the voices occur and what the voices say needs our attention, not who says it.
15 Then the seventh angel blew his trumpet, and there were loud voices in heaven, saying, “The kingdom of the world has become the kingdom of our Lord and of his Christ, and he shall reign forever and ever.”