Synesthesia

According to Science Daily, synesthesia is “a neurological condition in which two or more bodily sensations are coupled.” There are currently more than sixty known types of synesthesia , a recent research  demonstrates that “synesthetes have stronger connections between different brain areas…Those connections lead to a triggering effect, where a stimulus in one part of the brain would cause activity in another.” Psychology professor Jordan Peterson explains, “The normal person classifies an object, and then forgets about it, even though that object is much more complex and interesting than he or she thinks. The creative person, by contrast, is always open to new possibilities.” Which means that creative individuals remain in contact with the extra information constantly streaming in from the environment, Other people's brains might shut out this same information through a process called "latent inhibition" - defined as an animal's unconscious capacity to ignore stimuli that experience has shown are irrelevant to its needs. Through psychological testing, the researchers showed that creative individuals are much more likely to have low levels of latent inhibition

So Kandinsky’s ability was definitely guiding him through the manifestation of sounds to choose the concept of his paintings but how is it possible to see sounds exactly ?

Asking this question is kind of tricky because people with this type of anesthesia don't see sounds in the shape of musical notes like you probably imagined while reading the question , they rather "feel" it, for example hearing the "G" note can give the impression of green, it feels green, looks green and sounds green,(the color is inside him, not in the air , not inside his head) same goes with the other notes, in other words the forms and colors, the musical notes are released from theur external appearences of things and are reinscribed within the pathos of life hence the rise of the theory of abstraction (elements).