The Auditory modality consists of those neural networks that extend between the receptive (ear) and expressive transducers
Speech sounds are the medium for language in this modality.
In the history of human development, there was a time (before writing) when the Auditory modality was the only channel supporting language.
All of the cultural amenities for society (religion, mores, government and history etc.) had to be transmitted over generations by means of speech, in stories and song.
Because language is one of the few human skills that improves with age, these important functions were typically left to the older members of the society.
The Visual Modality includes those neural circuits between the receptive transducer (the eye) and the expressive transducers: the mechanisms for writing, Sign Language and Pantomime.
For most of us, graphemes (written letters) are the the medium for this channel of language.