Interesting points about voices and visions, Wrose and Patty. I would think a lot of fasting and other self-mortification could contribute. And just think how easily we mistake noises or shadows for something they're not.
Regarding spirit guides - I would love to know if these people ever say anything more substantive than, "Your loved one is ok and wants you to know they love you." sort of thing. BKSjones - any stories from your uncles about real specific communication from the other world?
I guess I would attribute it to a very active imagination. It's real easy for us to contemplate something and then it seems like a voice is answering you but it's really just your mind putting it into the form you want to hear.