Audiobooks on Oasis 2017

[johnnyb]

Hi folks,
for quite some time now I've been content with just reading my books and neglecting the technical side of things. However, I was recently tempted and in fact did acquire a Kindle Oasis 2017 with 32GB memory for a very modest price (Like New, 145USD before tax).
Now I am wondering what to do with all that memory as I couldn't possibly fill it up with books alone. However, I also have my audiobook purchases spread across multiple sources (I was allowed by Amazon to open a second account in Germany to be able to operate a Fire TV stick with Prime there, since this is where my German Prime runs, I also got a few audiobooks there from promotions). Is there a way to consolidate these on the device? Or alternatively, is there a way to run downloaded Audible books (and perhaps even ripped CDs) on the Kindle, perhaps by adjusting the file format?
If not, no problem, but being able to do so would really increase the value.

Thank you for your input!