Not much more to tell, really. It is mentioned many times in Castaneda books, which say that it is the voice of an "inorganic being" that can interact with the dreamer, in both positive and negative ways.

Anecdotal evidence for a "voice-in-dreaming" exists on this website. I mean, I've certainly read about it a few times on here, but I can't point you to the exact threads.

BTW The "voice" can be quite good at answering questions, so maybe next time you could ask it what you want to know about it.