Reading aloud
We have a tutorial on this site.
FBReader has no own speech generator. Instead, it uses text-to-speech engines installed in the system.
On Android, you can install different engines and voices. Some of them are free; some others have to be purchased. Look to the system text-to-speech settings and for engines in your software store.
That might be related to energy-saving settings. On many modern Android devices, the system tries to stop background processes to save the battery.
The solution depends on your device model. Some manufacturers allow to include an application to the list of “exceptions” from the power optimisation mechanism. If your device has such an option, you need to include two apps: FBReader Premium and the text-to-speech engine.