We are not told. Is this a reference to the Lamb whom John had earlier seen “between the throne and the four living creatures and among the elders” (Revelation 5:6)? Perhaps, but it is unlikely since the Lamb had since moved to take the scroll from God’s hand. It is sufficient to say that this is a heavenly voice speaking with knowledge and authority.
6 And I heard what seemed to be a voice in the midst of the four living creatures, saying, “A quart of wheat for a denarius, and three quarts of barley for a denarius, and do not harm the oil and wine!”