Here's how that works: Audible is a division of Amazon, and they've illegally obtained a monopoly over the audiobook market, controlling more than 90% of audiobook sales in many genres. That means that if your book isn't for sale on Audible, it might as well not exist.
But Amazon won't let you sell your books on Audible unless you let them wrap those books in "digital rights management," a kind of encryption that locks them to Audible's authorized players.
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