A voice (phōnēn mian). For mian as indefinite article see Rev 8:13. Accusative case here after ēkousa, though genitive in Rev 8:13, a distinction between sound and sense sometimes exists (Act 9:7; Act 22:9), but not here as the words are clearly heard in both instances.
From (ek). “Out of the horns.” Note triple use of the genitive article here as of the accusative article with this identical phrase in Rev 8:3 (“the altar the golden the one before the throne”).