I don't know if they are the best numbers, but https://wordsrated.com/audiobook-sales-statistics/ has some break downs. The digital market has clearly seen a shift in the last couple of decades. And it seems safe to say that it has driven a lot of the growth in publishing and other related metrics.
My assertion is that publishers used to give higher cuts to the authors because they didn't care about such a small portion of their sales. More, their long term investments in printing made it such that they couldn't scale out audio books nearly as effectively, so they had no incentive to build out that market.
Even dumber in this debate, the fact that Audible puts their files in DRM is almost certainly at the demands of the publishers. You can click through most publishers to see they still want to charge 25+ for audio books that you can get for 1 "credit" on Audible. Credits being about 11 bucks, and I get why publishers would want to keep those files restricted to try and discourage people from using Audible.