I do love your point because I felt like synesthesia might fit in my first comment but was not sure how to phrase it.

By vowel continuum I really only meant all vowels between A and U; basically all the vowels a human is capable of making. Even though you might not feel the shapes, your brain certainly does. There are videos that show the folds as someone speaks and you can see those shapes are deterministic.

I've realized those shapes while learning polyphonic singing. By doing the usual exercise of going from one vowel to another you begin to understand there are specific shapes for vowel sounds: O (in portuguese) is very "bouba" (round) while I is very sharp like a spear; kiki.

As for the comment on consonants, it was not made by me but I do like it: https://news.ycombinator.com/item?id=50094626