The truth is, it seems that God spoke to the prophets in human voices, for example, the voice Mosses heard at the burning bush, was his father’s Amram’s, the reason little Shmuel ran to Eli the Cohen, when God called him, was because he used Eli’s voice, and I suppose, that God often is using our own voice, as the Rebbe says, we often prophesize, as God can be a thought in our mind, though because it is part of the babble in our own heads, we simply don’t realize!