What part of speech are non-human "interjections" like "oink" and "bang"?

As a spin-off from this comment:

If a human exclaims something like "ouch!", I believe it's considered an interjection.

But if a pig exclaims "oink!", what is the part of speech?

And if a bell goes "bong!", what is the part of speech?

You could speak of "an oink" and "a bong" as nouns, but I mean in a context like

The man went "ouch!", the pig went "oink!" and the bell went "bong!"

Are interjections only for humans?

Answer

According to the Cambridge Grammar of the English Language, "the general definition of interjection is that it is a category of words that do not combine with other words in integrated syntactic constructions, and have expressive rather than propositional meaning." It seems to me that oink and bong fit that definition. Onomatopoeia is not one of the parts of speech listed in CGEL or any other grammar I'm familiar with.

This is, of course, limited to actual words. Environmental sounds (i.e., those you hear directly from pigs and bells rather than human descriptions of pigs and bells) are not language, and cannot be assigned to any lexical category.