John had earlier seen activities in heaven because he was there (e.g., Revelation 4:1–11). But in Revelation 17:3 he had been carried back to earth so as to witness Babylon on earth and her destruction (Revelation 18:21). From his place on earth John now hears voices from heaven. This explains too the reference to a “loud” voice.
1 After this I heard what seemed to be the loud voice of a great multitude in heaven, crying out, “Hallelujah! Salvation and glory and power belong to our God,