That the voice comes “from heaven” indicates that John’s stage is still earth, as it was since Revelation 12:12. This is where the Lamb and the 144,000 with him are pictured as being. Heaven responds to developments on earth, that is, to the Lamb standing on Mount Zion with the full number of the elect despite the fury of the dragon (and his beasts) directed against the offspring of the woman (Revelation 12:17).
2 And I heard a voice from heaven like the roar of many waters and like the sound of loud thunder. The voice I heard was like the sound of harpists playing on their harps,