How is it possible for us to associate sounds with meanings and physical shapes?

[Peter1986]

I have always found this quite interesting, and it seems like it could be a very common form of synesthesia.
One of the more famous experiments on this is the "Kiki/Bouba" test, where apparently as many as 98% associate the word "Kiki" with sharp, pointy shapes, whereas most people associate "Bouba" with soft, rounded shapes.
This probably has something to do with the way we shape our mouths while saying those words, but some other sounds are a bit harder to explain.
For example, the sound of a clarinet is often decribed as "stringy" or "reedy", and even though this shouln't really have anything to do with clarinet sounds, there still seems to be some sort of connection there, just like the sound of a flute is commonly decribed as "mellow".
I also remember when someone challenged me to guess what colour the word "avis" belonged to, and for some reason I immediately just knew that it was probably either blue or white, and certainly not red - and I was right, it was indeed either blue or white (I cannot remember which one, but it was one of them).
And I could make these conclusions even though I had never heard that word before --- "avis" just felt blue or white to me.

There is also that word "chartreuse", which a lot of people seem to mistake for red or maroon, even though it is actually yellowish green.
I think the reason for this is because "chartreuse" really isn't a particularly fitting word for the yellow/green spectrum, since it just somehow has a more reddish tone to it - I have suggested the word "calleuse" a few times, since I feel that that word has a much more green/yellow sound to it, but officially the right word is "chartreuse", which to me, and apparently a lot of other people, seems to be more associated with red.

Some other words have more logical connections though, like for example the word "quick" is a short word with short syllables, and hence the word itself can seem quick to say, and the words "large" and "tiny" are spoken with large and tiny mouth movements, respectively.