I am reading the book of Roy Harris "Rethinking writing" and see his arguments in favor of autonomity of writing from speech.
There could be no complete isomorphism between any system of visible marks and any system of sounds for a quite fundamental reason already mentioned above; namely, the incommensurability of the sensory modalities involved.
...the basis on which visual configurations are identified and distinguished has no phonetic counterpart at all.
I will not agree: there can definitely be established some kind of morphism between visual and audible just because we have systems of visualization, illustrations of sound waves. I am sure there are systematic relations between signifieds and signifiers in a vast majority of cases, which bears out the possibility of establishing iconic (diagrammic) - in Peirce's sense - signs - what exactly is needed in the case of a system of sounds and a system of written figures.
So, do those visualization means tackle the task? Are the two perceptual modalities really incommesurable?