Chris Anderson and his readers with very insightful comments discuss DRM, piracy, (viral) marketing, digital content, cross media packaging, economics and more in this post on the Long Tail blog. I really digg the last quote below with the innovative audiobook bundle.
"As Tim O'Reilly puts it, obscurity is a far greater threat to authors and creative artists than piracy.
In principle, I'm in favor of free. Free digital products can be great marketing for a superior (or at least complimentary) analog version. In music, free digital songs can create demand for concerts. In my case, a free ebook can created demand for the actual print book or my speeches. Cory Doctorow gives away his books and says that it's a clear net positive, and even business authors such as Seth Godin have tried it for the promotional phase of a book release with success.
But an audiobook is not as clear a case for free as an ebook. Perhaps the best compromise would to be to have a code printed in each hard-copy version of the book that would allow the buyers to download a free audiobook, so they could choose whether to read the book in print or just listen to it in the car, saving the hard copy for reference. This would cost practically nothing and presumably there'd be very little overlap with the dedicated audiobook buyer, who usually don't buy the hard-copy version."