I always thought the most helpful evidence (that was once pointed out to me) that sound-symbols are arbitrary is proof that even onomatopoeia varies from language to language. So even the sounds that we claim represent most directly are not necessarily similar from language to language. Here are some good examples:

http://mentalfloss.com/article/51996/12-onomatopoeias-around-world

https://vimeo.com/25215616

(This video is also interesting in that the more complex the animal's sound, the more dramatically onomatopoetic words vary from language to language. A cat's "meow" sounds almost the same in any language, whereas there are many different ways of representing a dog's bark, or a rooster's crow. I guess "meow" is the closest humanity can get to non-arbitrary language. Meow = Truth???)