We are not told from whose mouth (or perhaps mouths) this voice came. All we need to know is that this loud voice was heard in heaven.
The creatures of heaven just witnessed the dragon’s defeat at the hand of Michael and his angels
and saw that he was thrown
out of heaven to the earth. Earlier, of course, the creatures of heaven had witnessed the child’s entry into heaven (Revelation 12:5) and how he had taken the scroll from God’s hand (Revelation 5:8). When this same Christ authorized the blowing of the seventh trumpet, these heavenly creatures had voiced, or witnessed, the declaration of Revelation 11:15.
10 And I heard a loud voice in heaven, saying, “Now the salvation and the power and the kingdom of our God and the authority of his Christ have come, for the accuser of our brothers has been thrown down, who accuses them day and night before our God.