Recently my clip+ had a hiccup and froze in the process of shutting down. Later I found bookmarking would not work with audiobooks, making it about useless. Here is how I fixed it,
in case others encounter this.
I tried a million things like dropping the firmware in root for a rebuild, forcing a rebuild of the song database (slow… I have a 32g card, altho the database song count is not maxed out), and deleting Mtable.sys. Then I noticed it would only regenerate a zero size mtable, so after that I dropped the firmware in again and it finally rebuilt things right!