The speaker is not identified and so we are not to speculate who it is. This voice, however, comes in the context of the messages of the three angels and the encouragement their words gave to the saints. Those angels had spoken on earth words that came from God—second hand, if you will. So it is striking that the fourth message as printed in Revelation 14:13 does not come indirectly, via another angel, but directly from heaven itself. That speaks to the increased importance of this message as well as to the need for earthlings to hear it immediately.
13 And I heard a voice from heaven saying, “Write this: Blessed are the dead who die in the Lord from now on.” “Blessed indeed,” says the Spirit, “that they may rest from their labors, for their deeds follow them!”