I have a cd that accompanies an English textbook. This cd contains the audio of the conversations and the listening exercises. Can I call this cd the book's audio as in I don't have the book's audio?
The book's audio
will have the meaning that it is an "audio" recording of the entire book, as in an audio book. In your case the CD is
the book's supplemental audio CD
which is a CD included in the course work of the book. The same would be true of
supplemental flash cards