Student or Learner
All paralinguistic messages provide much useful information about the speaker; information which is either consciously or subconsciously received. In most cases people appear to interpret the messages appropriately, except where there is interference because of prejudice.
I think "except the case where" is the full meaning. "the case" is omitted here.
"Where" can function as an relative adverb even for concepts, not only for places, right?
Yes, the case could be inserted with the same meaning.
Write a relative, not an relative.
I am not a teacher.