I found this link you might find interesting. This guy has a theory that robert johnsons songs were recorded too fast so he has slowed them down. He has two clips on his page. It sounds right to me. A guy I know says he heared a rumour that the record producer sped up the recording to try and make it more jazzy which was becoming popular at that time. What do you think.
allthumbs
http://www.touched.co.uk/press/rjnote.html