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.
Most people would rather die than think. Most people do... Bertrand Russell