Not audibooks but as a kid I had tons of radio plays on cassette. Are those only popular in Germany?
The most popular ones were about groups of 3-6 kids doing detective work or going on other adventures. The biggest one was ??? (The Three Questionmarks). I think there are still new episodes coming out today with the protagonists being adults by now. I even went to a live performance once. It was awesome.
But often TV shows got also turned into radio plays. More often than not they would take the audio from the show 1:1 and put it on cassettes. I had Ducktales, TMNT and a weird Playmobil one. For some of these I would actually see the original TV episode much later but would finally realise what the weird sound effects were supposed to represent.
The three investigators were continued in Germany only. It's still a huge thing thanks to the radio plays. The first ones were released in 1979 based on the original English novels. After the series was discontinued in the States, German writers took over and published new books. With the radio plays we're now at episode 250 with the original voice actors from back in the day who are now in their 50s / 60s.
Radio plays are not exclusive to Germany, but their popularity in Germany is 2nd to none. In Great Britain, The Hitchhiker's Guide To The Galaxy (Per Anhalter durch die Galaxis) started out as a radio play on BBC before it was published as a book.