Good morning, Georg:
I'm not Cornelius, but I will chime in. In the long run it does not matter. An audiobook is still a book so it could be either, and I really don't think PMI cares in the long run as long as you can provide evidence that you've completed the task. A podcast is clearly "online or digital media".
My advice would be pick one and be consistent when you fill out your PDUs. Personally, I've done only printed material as "Read" and all others (audiobooks, podcasts, etc.) as "Digital Media."
Harry
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Harry J. Elston, Ph.D., CIH, PMP