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
The voice comes from the golden altar upon which the prayers of the saints were offered and from which they ascended before God (Rev. Rev. 8:3-4+). Mention is made of the altar to associate the judgment of the sixth trumpet with the petitions of the saints previously offered upon the altar. The voice could be that of the angel who offered the prayers of the saints upon the altar, but more likely, is the voice of the Father because that which is commanded requires the utmost in authoritythe release of the bound angels.2 The horns of the altar are intended to signify mercy, for by running to the temple and clinging to them a man could seek sanctuary (1K. 1K. 1:50-51; 1K. 2:20). But now, a voice of judgment comes from the horns of mercy.
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.