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

[Skyblade]

Hi, synesthete here.

You answered your own question in the first sentence.

Not really, I simply suggested that as a possilbe explanation and encouraged a discussion (hence the "seems like" and "could be" parts).
The issue there though is that synesthesia is typically somewhat uncommon, so it's kind of bold to confidently assume that this is the explanation.

Anyway, I have noticed that I tend to often agree with Tolkien's choices of words for his invented languages in the "Lord Of The Rings" legendarium.
For example, for some reason I always felt that "rokko" is an incredibly fitting word for "horse", and I believe his Quenya words for "light" and "dark" are "calë" and "mornië", respectively.
I usually often disagree with people's choices of words when they invent their own languages, because their words typically feel "off" or poorly chosen, but Tolkien almost always nails it to me.
"Calë" has a much more "bright" and "shining" sound than "mornië" IMO, and vice versa, and I believe that he had a lot of knowledge about this since he had a huge passion for languages, so he probably knew a lot about this apparent "sound synesthesia" thing adn which words to go for.

His choices of words for colours is also quite interesting - his Quenya word for "red" is "carnë", and I could never imagine a word like that being used for "green" or anything like that.
His Quenya word for that colour, "laiquë", feels much more fitting.
These are some examples of words that probably cannot be easily explained - they just kinda seem to fit their meanings in some weird ways.

I find these kinds of sound/object connections very fascinating, so I could go on forever like this.

Well, when it comes to Tolkein, it is important to remember that he was a professor of linguistics. So, his made up language was actually constructed by someone who knew and studied the structure and makeup of languages and words.

Some of his words weren't even invented by himself, but rather, reformed. For example, I believe "ent" was originally found in Proto-Germanic cultures, among context typically used for giants and ettins, but without a clear definition of what it referred to. So he created a new giant-type creature for whom the word would apply.

Tolkein's love of language and words was behind a lot of his works (as you will see very clearly if you read The Silmarillion).