Can I have more than one child content type for a book content type?
Yes you can create two different Content types for your chapter nodes, just like you have different Content types for your book nodes. 

The "Parent-Child" relationship you describe is usually accomplished with Entity references. 

Add Entity reference fields to your chapter_type and audio_chapter-type and configure them to reference text_book_type and audio_book_type nodes respectively. 

Then when creating a chapter node you'll be able to choose which book node it belongs to. 

You can use Views to list all child nodes on a book page through that reference field, if you need such a list.