The terms on which Amazon want to offer Audiobooks are their business, but as I consumer I certainly wouldn't expect them to affect any downline incomes.
Yes, they do actually. They organize sales and have temporary reduced prices sold to customers and paid to suppliers. That’s why some cereals go on sale and others don’t. It’s not like they decide to take a loss only on Kelloggs cereal. The discount is because they pay less to manufacturers.
Similarly for audible, the contract stipulates this. So I think it would be wrong if Audible unilaterally decided to not pay authors. But they didn’t do that, they have a contract, signed by authors that lets them put books on sale and pay less.
No, they negotiate those things ahead of time. It does t matter though, as there’s a huge difference between an item that is infinitely reproducible and a commodity that has per-unit production costs.
For the record, I also disagree with your blanket claim that supermarket discounts are negotiated ahead of time. Some are.
Not necessarily because of sales, but one of my parents' neighbours (he was previously a fairly senior guy at a flour manufacturer) spoke from experience of big supermarket retailers "renegotiating" unit price after the fact (and after delaying payment until duress) under the threat of removing all their products from sale if they don't accept.