Why is this the case? First of all, how can two such diverse modes -- sight and hearing -- equally create the thing called "art?" Or, perhaps more to the point, what is art that it can express itself in two such diverse modes? Why are a painting and a symphony both called art? And why are the other senses excluded?
One of the classic definitions of art is that it combines, wholeness (integritas), harmony (consonantia), and radiance (claritas, which is similar to Plato's "splendor of truth").
Thus, painting and music can obviously embody wholeness and harmony, but it is difficult to imagine how the other senses could do so. For example, touch is inherently fragmentary; one cannot "touch the whole," nor can the fingers perceive radiance. And no one imagines that truth can be tasted or smelled (except in a subtle, analogical manner).
Let's go back to Joyce, who is speaking through Stephen: "An aesthetic image is presented to us either in space or in time. What is audible is presented in time, what is visible is presented in space."
The "mysterious instant" of aesthetic reception occurs when "the supreme quality of beauty... is apprehended luminously by the mind which has been arrested by its wholeness and fascinated by its harmony." There is in "the silent stasis of aesthetic pleasure, a spiritual state very like to that cardiac condition which... [is] called the enchantment of the heart."
Perhaps vision conveys the image of eternity, while sound is the moving image of eternity. As Schuon says, aesthetics is "the science of forms," and music presents us with temporal form, or architecture in motion. But the form must convey what is non-formal, i.e., the supra-formal light -- and truth -- from another world. It is limitlessness expressed by a limit, or divine radiance expressed through wholeness and harmony.
Interestingly, Schuon writes that "ignorant and profane aesthetics places the beautiful -- or what its sentimental idealism takes to be beautiful -- above the true..." This leads to idolatry of beauty, and of "art for art's sake." But beauty should be for truth's sake. If it is not subordinate to something higher, it will be appropriated by something lower.
Sri Aurobindo says something similar in a letter to a disciple, that through sound or image, "in a moment mysteriously, unexpectedly, there is a Presence, a Power, a Face that looks into yours, an inner sight..."
However, "so long as one is satisfied with looking through windows, the gain is only initial; one day one will have to take up the pilgrim's staff and start out to journey there where the Reality is forever manifest and present."
Or, one must follow the light to the sun and sound to the moon, for "in a certain sense, the sun makes known space and the moon, time" (Schuon).