Clairaudience is the term used for psychic and mediumistic hearing abilities and means "clear hearing". A clairaudient person receives psychic and/or information from spirit in the form of subtle sounds, words or ideas that are perceived and interpreted through the hearing centers of the brain.
There are two main ways to hear clairaudiently, one in it's simplest form and most common is hearing loud thoughts and ideas internally (subjective) just like when we talk to ourselves. The other type is hearing the voices externally (objective) as if the spirit person is actually in the room with you, I've experienced this a few times and is often spontaneous, so when developing Clairaudience don't expect to hear physical voices all the time as you will be disappointed. Some people who develop strong clairaudience can be affected with the day to day noises such as traffic and loud music, sometimes loud sounds can make you feel physically sick.
Many people have a degree of Clairaudience, but do not recognize it as such because like so many other psychic/spiritual abilities it has always been there. We may have always received words, thoughts or sounds of inspiration inside our mind without ever questioning their source and their precise nature. Numerous good ideas may have originated as clairaudient messages or as information perceived through one of our other senses.