^11:15 The seventh angel sounded his trumpet, and there were loud voices in heaven, which said:

“The kingdom of the world has become
the kingdom of our Lord and of his Messiah,
and he will reign for ever and ever.”

The seventh angel sounded his trumpet,

This is the last "trumpet" judgment (Revelation 8:2,6). It coincides with the third woe (verse 14) and contains the portion (Revelation 11:16 - Revelation 19:11) with the seven bowls.

and there were loud voices in heaven, which said:

Not clear if these are angelic or saintly or both. There is no context here other than the previous announcements (Revelation 10:1,3) were by a strong angel with a loud voice.

“The kingdom of the world has become
the kingdom of our Lord and of his Messiah,

Kingdom(s) are what Satan offered Jesus at temptation time (Luke 4:1-13). Satan even admitted 'they were handed over to him' so he got them from someone else. Now that "SOMEONE" has acquired the 'Kingdom of the World' again. The world has to be reacquired from the devil such that he will not have any authority there when he is kicked out of heaven (Revelation 12:9).

and He will reign FOREVER AND EVER."

There will not be a possibility that another "fallen angel" will acquire the Kingdom of the World. This is a FINAL solution; the end of Satan's rule. The interpretation of Daniel to a worldly king's dream (Daniel 2:36-44-45) indicates after this king there would be other kingdoms, and then a FOREVER kingdom. Psalm 2:1-6, Psalm 45:6-9, and Revelation 19:6 talk about this 'FOREVER and EVER' kingdom.