The primary auditory cortex is the area of the cerebrum that serves as a biological processor of sound signals that represent pitch and volume.
Anatomically, it is part of the temporal lobe, in the cerebrum. Sound signals reach perception when they are obtained and processed by the cerebral cortex. Dysfunction of the Primary Auditory Cortex leads to a loss of any recognition of sound stimuli, but an ability to respond spontaneously to sounds remains intact. Most probably it is due to the sub cortical data processing in the auditory brainstem and midbrain.
The dominant known criteria for the organization of neurons in the auditory cortex is their sensitivity to signals according to their frequency of sound that they represent. Tonotopic map means the order of sensitivity neurons to frequency in a defined area of the auditory cortex.
There are many auditory areas, that can be distinguished anatomically one from the other. Known activities of the auditory cortex are: identifying and discriminating auditory "objects" and placing of a sound generator in space.
The hearing system receives multiple sound stimuli at every point of the time scale, and have to process all of them simultaneously.
The role of the auditory system is to purify the signal by discriminating the main sound signal from the background noise.
For example: two people are talking in a farm. The sound of speech is the main signal and the other sounds such as the blowing of the wind, is the background noise. Suddenly, another component is enriching the gallery of sounds: a hissing of a snake. Within seconds the priorities are changing. The hissing becomes the main issue.
How it is done? We have theories, but not enough evidence.
The terminology of primary auditory cortex that is used today relates to the area that is very close to Brodmann areas 41 and 42. It is placed in the back half of the superior temporal gyrus and the lateral sulcus as the transverse temporal gyri.
The anatomic term for that area is Heschl's gyri. There are also more areas that receive and process auditory signals in the frontal and parietal lobes.
When every instrument of the symphony orchestra or the jazz band plays the same note, it is possible that many of the audience are sleepy and do not notice that the pitch of every instrument is the same, but the quality of every sound is different.
The Musicians in the two bands are fully awake (usually) and perceives every note as having the same pitch. The neurons of the auditory cortex of the brain are able to provide a relevant response to pitch.
More information about the location of the auditory cortex of the brain can be found on Anatomy illustrations.