Yes, it is possible to hear a spell being cast. If a person has sensitive clairaudiance and telepathy that energetic link can be felt and sometimes heard.

By mainly mental I am speaking of the mindset that the person being hexed or cursed has. If it is a limiting belief mindset, prone to victimization and projection, with a weak will and mind it leaves the psychic doors wide open for infiltration from all sorts of unpleasantness.