The next time I went in, he was laying on his bed staring at the iPod speaker doing nothing else and lost in the "book." He can't get enough. He's listening to Half Magic by Edward Eager, a book first published in 1954!
I loved 'Half Magic' as a little girl.
I would limit the audio books about the same way I limit other books. And I do, occasionally have to draw the line when homework or company come first.
My brother used to read during playdates. I used to watch the other little boy sit 3 inches from my brother - watching him read for hours. That looked kind of weird to me. At our house when DS was younger it was only allowed during TV/reading breaks for long playdates.
I love it when DS get all indignant about me limiting his reading, really, 'How could I? His friends parents pay them to read!' etc.
If you feel the need to 'enrich' the experience, you could try having DS retell the story, or use the '6+1 writing traits' to talk about word choice, etc.
Smiles,
Grinity