Froskur: Looking through what Librivox has for adventure, science fiction, and children's books, here's what I found:
These science fiction authors are fairly recent and not so difficult as many others: Poul Anderson, Marion Zimmer Bradley, Ray Cummings, Henry Kutner, Andre Norton, Alan E. Nourse. H. Beam Piper. As you can see, Elric and I listed some of the same authors. I went through the librivox science fiction catalog and can list all the most recent authors, if you would like, but many of them are rather difficult reading.
You might try Zane Grey, who wrote adventure novels about the American West.
Among children's authors: Jim Kjelgaard, is fairly recent. L. Frank Baum (The Wonderful Wizard of Oz, and many others) wrote his books 100 years ago and more, but his style is not complex, and the stories may carry you along fairly well. Or is that too easy? Reading children's stories is a very good idea. Perhaps Helen (skyblueteapot) will have some good suggestions for you.
I agree with Elric: Mark Nelson is a very good reader, and he speaks clearly. That is just what a language learner needs. Maybe Edgar Rice Burroughs' style is a little old fashioned--but maybe not. He wrote great adventure stories, and many of them are on librivox. So if you like him, there is plenty to choose from. Currently I am reading a Russian audiobook of A Princess of Mars. I certainly don't want to discourage you from trying to read it. (Elric, I'm on chapter 17, but I read the chapters over and over, so I move slowly.)
Let us all know what you like or don't like, Froskur.