I have 20 audiobooks in the Audiobooks directory, all 20 loaded together at the same time (486 files - Total weight :1.19 Go ). They work OK.
I wanted to add 5 more audiobooks (119 files - 392 Mo). ONLY ONE of them showed in the audiobooks directory (all 25 show in the Folder directory).
I had to remove all of them and reload the 25 together at the same time to have the 25 show in the audiobook directory.
That's a big bug, gentlemen !
Buying this crap firmware was a big mistake of mine.