Any reason or ideas out there why aura refers to something that is detected using either olfactory or visual senses, but aural refers to the sense of hearing? Why the change in sense, so to speak?

They have different roots. According to the Online Etymological Dictionary, aura come from the Latin and Greek aura, meaning "breeze". Aural comes from the Latin auris, "ear".

Interesting that ear and air have had similiar sounding words for some time.