I personally don’t think that a message like that would make much of a difference because they won’t understand the nuances of the situation that’s occurring. All they care about is eyeballs on their product, and avoiding association with unsavory things.
Because of that, I would think that a better tactic would be to talk about how you saw their advertisements in a subreddit with inappropriate content and ask if that was intentional. Asking if it was intentional is a good tactic because it turns it from a report into a question. Even better, word it like “Do you have any comment?” as if you’re a reporter writing a story. You don’t have to say who you’re writing “a story” for, you can just say that you’re an independent journalist working on a piece and wanted to get a statement on their choice to place advertising on a service that serves their advertisement next to (give some examples)
With that tactic, you can also skip the standard feedback forms and attempt to contact their advertising arm or PR department.
I notice that you specifically mentioned audiobooks, but if you’re interested in written ebooks, check out Standard EBooks
They take public domain books and run them through a detailed process of editing and typesetting them to create beautiful versions.
Personally, I’m a big fan of The Personal Recollections of Joan of Arc by Mark Twain. There is an audiobook version on Librevox too!