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

Solution 1:

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.

Solution 2:

I think that the "non-human" sounds described here are best termed as simply onomatopoeia. Sound effects, whether anthropomorphized into something the non-human thing "says" or not, are just sound effects; neither the pig nor the bell is "exclaiming" or "interjecting" except as the result of human attributes we ascribe to them.

Solution 3:

This is not really a problem, if one keeps in mind that only humans use language. It follows that:

• if any thing makes noise, it's not speech sound.
• if any person speaks or writes those sounds, they're speech sounds.

Thus, if anyone reports sound in speech, it's all speech sounds. That's how easy it is to tell the difference.

As to what Part of Speech the words are, you can call them anything you like, and it won't matter a bit. Knowing an official "Part of Speech" to label a word (or anything else) contributes precisely zero to one's knowledge of English grammar. As I point out in the post linked above, not everything they teach you in school is correct. Alas.

Real parts of speech (nowadays they're called Grammatical Categories) are simply those categories that are necessary to describe the grammar of a language, and they vary greatly -- like vowels or syllables -- from one language to another.