To listen to your plex audiobooks on mobile, you have a few options and they differ based on your mobile device OS:
To access your Plex audiobooks on any of the below apps, simply log into your Plex account and choose your desired server.
For both iOS and Android, Plexamp is the official Plex audio streaming app. Basic features are free to all, but to unlock premium features requires a one-time $5 purchase. If you purchased Plex Pass, the full app is free to use, including premium features. Links included below to download from your preferred store.
The best free app for iOS is Prologue. Prologue is free to use for the basic features, which provide the following:
Stream audiobooks from your Plex server anywhere in the world
Remember playback positions across all your audiobooks
Use variable playback speed
Set custom sleep timers
Use voice boost feature
Basic Apple Car Play integration
However, for a $6.99 one-time purchase, you can unlock more advanced features.
The best free app for Android devices is Chronicle. Chronicle supports the following features:
Syncs audiobook progress on device and across devices via Plex server.
Supports various file formats including mp3, m4a, m4b, and FLAC
Adjustable playback speed
Auto-rewind function
Sleep timer
Skip silent audio feature
Basic Android Auto integration