A.T. Robertson Commentary Revelation 9:13

"And the sixth angel sounded, and I heard a voice from the horns of the golden altar which is before God," — Revelation 9:13 (ASV)

A voice (φωνην μιαν). For μιαν as indefinite article see 8:13. Accusative case here after ηκουσα, though genitive in 8:13, a distinction between sound and sense sometimes exists (Acts 9:7; Acts 22:9), but not here as the words are clearly heard in both instances.

From (εκ). "Out of the horns." Note triple use of the genitive article here as of the accusative article with this identical phrase in 8:3 ("the altar the golden the one before the throne").