the sixth angel sounded
This is the sixth angel of the seven which were given trumpets (Rev. Rev. 8:2+). When he sounds, the second of the three final woes is brought forth. See commentary on Revelation 8:13.
Each time the trumpet sounds, the judgment which follows is more severe than the previous one. Long ago Isaiah had prophesied that when thy judgments are in the earth, the inhabitants of the world will learn righteousness (Isa. Isa. 26:9). No doubt, therefore, each succeeding judgment will uncover a few reluctant learners, but those that remain unconverted are still more stubborn and thus each visitation must increase in severity.1a voice from the four horns of the golden altar
Shockingly, from the altar associated with mercy came words of judgment. God is a merciful, gracious, compassionate God, yet His Spirit shall not strive with man forever (Gen. Gen. 6:3). When this trumpet judgment occurs, the time for mercy will have passed; the altar of mercy will become an altar of judgment.3
3 John MacArthur, Revelation 1-11 : The MacArthur New Testament Commentary (Chicago, IL: Moody Press, 1999), Rev. 9:13.