iTunes displays two quantitative measures for the "quality" of the encoded audio for an audiobook, the bit rate and the sample rate. The bit rate is the best pragmatic measure of quality, it basically describes the amount of bytes it takes to record a second (or whatever) of audio. More bytes = higher quality, all other things being equal (which they never are, but let's not get complicated). Skip sample rate for now.
As @MrMojo mentioned, Audible has multiple formats, 2-4 and "Enhanced". (Format 1 was discontinued, it dated to when 64 megabytes would be a high capacity player.) Apple only sells audiobooks encoded in one format, recognizing that (most) people don't want to have to make decisions about which format to choose.
In terms of the quantitative measures of quality, here are the specifics of the three highest quality formats between both Apple and Audible:
As you can see from the differing sample rates, while Audible is indeed the supplier of Apple's audiobook titles, they don't supply the audio file directly. Apple's audiobooks have a different sample rate (which probably doesn't affect quality perceptibly), and they use a different encoding format.
In terms of more subjective measures of quality, the first two formats in the list are roughly comparable, and they are both "more than good enough." But if you have hard drive space and capacity on your listening device, Audible Enhanced is noticeably more crisp and clear, especially with books that have musical passages (e.g., mood music in between chapters). Enhanced format is relatively new, within the last year or so (my oldest Enhanced audiobook is dated 3/9/2009).
Neither Apple nor Audible charges different prices for audiobooks based on the encoding format or bit rate. It's all about how much the audiobook cost to produce, which is mostly related to the length of the book, and about how many copies are expected to be sold. If you look at more than a few titles, you'll see an almost perfect correlation between price and listening time. And if you compare between the iTunes Store and the Audible site, while Apple's prices tend to be a few dollars cheaper for an individual title, if you sign up for one of Audible's subscription plans, their pricing drops dramatically, to ~$13/book, by far the best deal in high quality audiobooks. (And they run regular member sales, where prices can drop as low as $5/book, truly a steal.)
Pricing has
nothing to do with the encoding format or bit rate quality.
while Audible is indeed the supplier of Apple's audiobook titles, they don't supply the audio file directly
Actually, yes Audible supplies the completed file to Apple and it is created by Audible with iTunes producer, which Apple gives to those who supply Apple with audio.
It's just as I suspected that iTunes provide only 1 bitrate, 32k, for their audiobook versions while Audible still provides 4 versions (yes even the lowest bitrate level 1 is still offered), the best being the Enhanced version @64k, which is my preference. The Enhanced version, available for about 1 year now, is now available for most, if not all, newer audiobook releases but not the older ones!
Guess I'll continue buying audiobooks from Audible and probably won't ever buy from iTunes since I prefer the higher bitrate versions as it has better sound quality and you don't need to increase the volume level so much to hear the recordings!
Too bad iTunes "refuses" to allow the buyer to choose the different bitrate formats.
