Are you checking the Playlist files with a text editor, or with MC? Because if you check them using MC, by opening them, MC will show them empty if it can't find the files.
The Playlists work fine for me. Use M3U with relative paths, as I find that works best. Using relative paths should ensure the files are found on the SD card in its caddy connected to the PC, or in your phone.
What phone do you have by the way?
Are you running MC26 now, and a recent version?
When syncing to a hard drive location MC shows a path settings for "Audible", which I haven't used but I assume applies to audiobooks. They will need the correct metadata to be recognised and transferred to the correct location, of course. A search of the forum for "Audio Books" or "Audible" will probably find information. Probably just need to set the [Media Sub Type] to "Audiobook", and add them to a Playlist to transfer of course.