BMG can't take you to court for burning a Sex Pistols CD for your grandma. But if you plan on putting these Max Brooks stories anywhere online that the public can access, you definitely need Max's permission.
The non-commercial nature of your idea should come across as nonthreatening to him, so you have that going for you. Also, agreeing on particular CC license protections would help. But you still may get a flat out "no". There may be a reason he hasn't released those stories in audio book form yet, who knows?
If he says no, you could go ahead and share the recordings with friends (like you do with your punk rock loving grandma), or read 'em to your kids for bedtime stories, but respect his wish to keep them from splashing the net.
It doesn't really work as 'fan fic' because you'd just be reading stories from his book, not building upon them as derivative works.
Another approach: Money talks. Ask him if you can buy the nonexclusive audio rights for the stories you want to run.
It's easy to steal creative content in the digital age, but stealing's bad, mmkay? Paying authors or at least asking permission mmkay, that's a good thing.