9:13 And the sixth angel sounded, and I heard a voice from the four horns of the golden altar which is before God,

And the sixth angel sounded,

The second WOE is commensing with the blast of the SIXTH TRUMPET.

and I heard a voice from the four horns of the golden altar which is before God,

John is not specific here as to the owner of 'a voice'. He is clear, though, that it comes from the four horns of the altar. Now the four horns on the altar of fragrant incense (Revelation 8:4) were touched by the blood of the bull sacrifice in the atonement of sin -- Leviticus 4:1-12.

A person could argue that this voice is from Jesus whose blood is being avenged; others believe it is God Himself since Christ is making intercession with/for the prayers of the saints -- Hebrews 7:25 and 1 John 2:1. Perhaps, this is why John is not precise as to whose voice this is.

In the Old Testament, the horns were a safe haven from judgment as in 1 Kings 1:49-53. By hanging on to the horns, a person could claim mercy. So, too, today, by accepting Christ's redemption blood, we find mercy. Anyone NOT hanging on will find judgment. Also note that this judgment is carried out in full view of God (not that He doesn't see everything) but meaning He fully commands it.