Same here, I have exactly the same symptom as enguerrand! IPhone 5 OTA upgrade, and I immediately found that the mp3 files already on the device, that had been converted to audiobooks, did not play. It flipped from one to the next in rapid succession and deleted each one. I hoped it was just something hinky from the upgrade process, so I then tried replacing the files after the upgrade had taken place, using both iTunes for Mac and for Windows, and the same thing happened. I also tried creating a playlist of the audiobooks and playing them that way. No luck. As I stared at the playlist screen on the phone, where it says "20 songs, 4 hours," I watched as the number of songs decreased from 20 down to 0 in about 5 seconds. The files are gone from the phone, not even a spotlight search can find them.
Before the files are marked as audiobooks, they play fine as songs both on the phone and in iTunes. But of course there's no way to increase the playback speed, which is the whole point of converting them to audiobooks in the first place.
I also noted that sometimes when the phone is still connected to iTunes, the audiobooks WILL show up in iTunes even when they don't show up on the phone. For example, on the phone I can't even select Audiobooks as a type under "More" because it thinks there are no audiobooks present, but at the same time I see Audiobooks listed in iTunes, on the phone under devices, and I can even see the titles. But if I attempt to play them, a little exclamation point in a triangle symbol pops up to the left of the title.
I'm really bummed about this, listening to spoken word audio files is about 75% of what I use my iphone for. C'mon Apple.