http://en.wikipedia.org/wiki/Mama_and_papa
http://en.wikipedia.org/wiki/Bouba/kiki_effect
I guess it gets arbitrary at complex enough levels though.
I'd hazard a guess that hok and krak have some component of instinctual/physical nature to them. I personally think that certain sounds are related to physical experiences or expressions of emotions. Obvious ones are surprise of "Oh!" with an open mouth.
"Hmmm" whilst thinking or concentrating, frowning and closing your mouth.
I'm currently watching my son learn to speak and his verbalizing seems pretty closely tied to his emotions at the moment.
"Oishii" means delicious in Japanese and it seems something that makes sense to say whilst you are smiling at enjoying your food. The long "ii" vowel to rhyme with the "e" of "she" in English.