I was also going crazy trying to figure out why some things worked and others were silent (audiobooks and music). I looked through all the file details, and those with a sample rate of 22.050 kHz files always ended up being silent. Convert them to 44.1 kHz and they'll work.
Unprotected file conversion (music and audiobooks)
1: Edit -> Preferences -> Import settings -> (i like to use MP3 Encoder) -> Setting-> Custom
2: Set Sample Rate to 44.100 kHz and set bit rate to whatever you want (usually i do 256kbps for music and 128kbps for audiobooks)
3: Hit all the okays.
4: Highlight files, then Go File-> Convert -> Create MP3 Version
5: If it was an audiobook, the conversion will make it music, so select those files, right click -> get info -> if it ask if you want to edit multiple items, click edit items -> options -> media kind -> audiobook
For protected audiobooks, like Audible you have to get past the DRM, supposedly this works but I havent tried it yet https://community.giffgaff.com/t5/Tips-Guides/Mastering-audiobooks-liberating-Au dible-books/td-p/18277766