Apocalypse Explained # 262
262. As of a trumpet speaking with me, signifies clear and manifest. This is evident from the signification of "trumpet," as being Divine truth manifested and revealed out of heaven (of which above, n. 55. A voice that is heard out of heaven with those who are in the spirit is heard usually as a human voice; but it was heard "as a trumpet speaking" because it was clearly and manifestly perceived by the angels, and what is clearly and manifestly perceived by them falls loudly into the hearing of the spirit; and this was done with John that his attention might be awakened, and thence his sight, lest anything should be obscure to him. This is meant by "the voice of a trumpet" in other places also (Matthew 24:31; Zechariah 9:14; Psalms 47:5; Revelation 8:2, 7-8, 13; 9:1, 13-14; 10:7; 18:22).
Matthew 24:31
31
He will send out his angels with a great sound of a trumpet, and they will gather together his chosen ones from the four winds, from one end of the sky to the other.