^10:7 but in the days of the voice of the seventh angel, when he is about to sound, then the mystery of God is finished, as He preached to His servants the prophets.

but in the days of the voice of the SEVENTH ANGEL,

Apparently, the voices of these seven angels sound like trumpets. That is the way their voices were given to them, Revelation 8:2. Otherwise, they praise and communicate by different means that we do not know or can hear.

The use of 'days' indicates it will take some time for the works of this SEVENTH ANGEL. Since the seven bowls are wrapped up in this trumpet, a person could say it will take the rest of the tribulation period.

when he is about to sound, then the MYSTERY OF GOD is finished,

What is this mystery? One may refer to 1 Corinthians 15:51-53 as the mystery. Others say this mystery is the complete ousting of evil and reclaiming the earth for Jesus and His bride (the true church) as in Revelation 11:15-18 or as in Daniel 12:5-12.

as He preached to His servants the PROPHETS.

Nothing unusual here...God's revelations to us have always been through prophets. Maybe that is why prophesy is the most desired spiritual gift (1 Corinthians 14:1-5). This is how we learn a bit of the mystery of God!