I work for an eBook distributor and we sell eBooks on Amazon and the refund policy is true there too. Amazon keep it's cut refund or not. They are the only store that does this to my knowledge.
We had an issue a few years back where we saw 1000s of refund every months, they looks really suspicious, the time between purchase and refund was always the same, same 10-20 adresses. etc. We informed Amazon that someone seem to be downloading everything and getting refunded. This stop afterward, but i'm not sure we got compensated for all the money lost on that.
Most store takes from 30% to 50% of the sale. The bigger you are/the more you sell, the better the condition you can get with resellers. So those % are different, but are probably close for everyone. (We don't sell on Audible, probably because the rate are too bad, so I don't know)
In general Audio books popularity is relatively recent. We are seeing a lot more from our publishers. Big resellers have move to their solution for Audio books, from using someone else, including Audible. New Audio specific reseller are doing in some specific (English) market, hopefully something change in the English market.