The passage does not tell us. Yet the fact that the Lamb of God could take the scroll from God’s hand (Revelation 5:7) and that all the heavenly creatures confessed that the Lamb is “worthy…to receive power and wealth and wisdom and might” (Revelation 5:12) makes clear that Jesus Christ is behind this voice. Whether it is his voice John hears or another’s (upon the Lamb’s command) is quite unimportant.
13 Then the sixth angel blew his trumpet, and I heard a voice from the four horns of the golden altar before God,