Re: is it impossible to not hear?

Great question!  It is possible for your brain to "ignore" information that 
it is receiving. An example of this is what is termed, "the cocktail party 
effect."  The following information is taken from a paper written by Barry 
Arons who works at the Massachussets' Institute of Technology Media Lab.  
The paper is available on-line at:

http://xenia.media.mit.edu/~barons/cocktail.html


"The ``cocktail party effect''--the ability to focus one's listening  
attention on a single talker among a cacophony of conversations
and background noise--has been recognized for some time. This specialized 
listening ability may be because of characteristics of the human speech 
production system, the auditory system, or high-level perceptual and 
language processing."


Dr. Arons describes a series of experiments in which "Subjects...listened 
to different spoken messages presented to each ear with headphones. In this 
configuration there is no directionality, there is simply a dichotic 
signal. The subjects had no difficulty in listening to the message played 
to one ear while rejecting sounds in the other ear. The recognition process 
can easily be switched to either ear at will. The subject could readily
shadow one message while listening, though with a slight delay.  ... Note 
that the subject's voice is usually monotonic and they typically
have little idea of the content of the message in the attended to ear. 
Virtually nothing can be recalled about the message content
presented to the other (rejected) ear, except that sounds were occurring."

Dr. Arons states,  "This is what might be called the ``what-did-you-say'' 
phenomenon.  Often when someone to whom you were not ``listening'' asks you 
a question, your first reaction is to say, ``uh, what did you say?'' But 
then, before the question is repeated, you can dredge it up yourself from 
memory. When this experiment was actually tried in my laboratory, the 
results agreed with our intuitions: there is a temporary memory for items 
to which we are not attending, but as Cherry, James, and Moray point out, 
no long-term memory."

In response to your last two questions, it is my guess that it is quite 
common for humans to be able to "shun their hearing".