> The "kiki" sound has more high frequency content than the "bouba" sound

And where did you get that from? In non-tonal languages the pitch conveys almost no information and people speak at very different ones (and for instance a male saying "kiki" will say it at lower frequencies than a woman saying "bouba" most of the time) so I find your affirmation very dubious.

> and it's no mystery why the brain associates one with the other.

Specialists of the field find that mysterious but some smartass on HN disagrees.