I came across this recently and found it fascinating.
If you guessed wrong, you're in the minority:
"There is a 95% to 98% likelihood that you paired the sharper sounds (obstruents) of the word “Kiki” with image on the left, and the softer sounds (sonorants) of “Bouba” with the softer image on the right." More.
Phonosemantics or simply, Sound Symbolism. Sounds have meaning, and as a corollary, visuals evoke sounds.
What I found interesting were the implications for Branding. Think about the connotations associated with the Nike 'Swoosh', or why the Apple logo works:
"It's very intriguing. Take Apple, for instance. A technology company offering what could be considered a very complicated set of products. Let's face it, lots of people are put off by technology. Yet, the name is familiar and safe. A ubiquitous fruit. And, in the context of your article, the word "apple" can be seen to begin somewhat sharply, but ends very sonorously. It sort of rounds off and tapers. In this way, perhaps, the name, its phonetics, and the logo all work to make a cold, technology company familiar, friendly and inviting. Subtle, but profound. Would it be the same if they'd called themselves "Orange"?". [Author].