I have published neither an app nor an audiobook, but my impression is that Apple both takes a far lower cut of the price and is considerably more transparent in how the advertised price translates to the royalty paid out. And Audible seems to have far more market control than Apple has.
Yet in HN discussions, people seem far more inclined to defend Amazon's practices than Apple's. I wonder whether the reason is that a lot more of them make their living off writing software than off publishing audiobooks (but maybe there is some other factor there that I'm missing).
[Disclosure: Apple employee & shareholder, not involved in store related activities, not speaking for Apple]
The difference here is that with Audible, the comparison is to traditional book publishing models. By all accounts, a 40% royalty (even on net sales) is pretty darn good by that standard.
Apple's 30% fee, in contrast, is compared to traditional software publishing models. Developers are used to being able to publish on Windows or web without paying commission to anyone.
Is Amazon's 75% or 60% unfair, or is it highway robbery? Not to mention what they actually pay instead of the contractual amount.