In Deuteronomy 18:4, Judges 6:37 and the following verses, this signifies that the good of innocence is holy to the Lord. (Apocalypse Explained 865, Apocalypse Revealed 623)

397. And the seven angels who had the seven trumpets prepared themselves to sound. (8:6) This symbolizes their being prepared and ready to examine the state of the church and consequent life in people for whom religion is faith alone.

The symbolism of trumpets follows from the statute governing their use among the children of Israel, which Moses describes in this way: Jehovah told Moses to make silver trumpets for summoning the assembly and for the setting out of the camps, and they were also to sound them on days of celebration, feasts, new moons, and over burnt offerings and sacrifices. Furthermore, when they went to war against enemies oppressing them, they were to sound an alarm with the trumpets, and then they would come into remembrance before Jehovah God and be saved from their enemies. (Numbers 10:1-10)

It can be seen from this what sounding with trumpets symbolizes. Here, that the seven angels sounding symbolizes an examination and exposure of the state of the church and its character among people for whom religion is faith alone, as is apparent from the particulars in this chapter and from the particulars in the following chapters up to chapter 16 inclusive, understood in their spiritual sense.

[2] From the ways trumpets were used among the children of Israel it can also be seen what trumpets and sounding them symbolize in the following places:

Sound a trumpet in Zion, and sound it in My holy mountain! ...For the day of Jehovah is coming... (Joel 2:1-2)

Jehovah will be seen over them, and His arrow will go forth like lightning; and the Lord Jehovih will sound the ram's horn... (Zechariah 9:14)

Jehovah shall go forth like a lion... (and) sound an alarm... (Isaiah 42:13)

...on that day a great ram's horn will be sounded, and those who perish in the land of Assyria, and those who are exiled in the land of Egypt, will come and bow themselves to Jehovah on the holy mountain... (Isaiah 27:13)

He will send His angels with the great sound of a trumpet, and they will gather together His elect from the four winds, from one end of heaven to the other. (Matthew 24:31)

Blessed are the people who know the trumpet's sound! They walk, O Jehovah, in the light of Your countenance. (Psalms 89:15)

When the morning stars sing together, and... the sons of God sound the trumpet. (Job 38:7)

[3] Since the soundings of trumpets had these symbolic meanings, and in the Israelite Church everything was presented concretely in accordance with correspondences and the consequent symbolism, therefore it also came to pass, when Jehovah descended upon Mount Sinai, that there were voices and lightnings and a thick cloud, and the sound of a ram's horn, loud, with the sound of the ram's horn growing and becoming louder and louder, so that the people in the camp trembled greatly. (Exodus 19:16-25)

Therefore it also came to pass that when the three hundred men with Gideon sounded their ram's horns in the campaign against Midian, then every Midianite man's sword was set against his companion and they fled (Judges 7:16-22). Likewise that twelve thousand of the children of Israel with holy vessels and trumpets in their hands overcame Midian (Numbers 31:1-8). Also that the wall of Jericho fell after seven priests with seven ram's horns went around the city seven times (Joshua 6:1-20).

Therefore we read in Jeremiah:

Sound against (Babylon) all around..., her walls are thrown down. (Jeremiah 50:15)

And in Zephaniah:

...a day of darkness and blackness..., a day of ram's horn and its sounding against the fortified cities... (Zephaniah 1:15-16)