When navigating using the book grid or list from a book without audio to a book with audio the Audio toolbar does not appear. Tapping the speaker on the top will make it appear.
The opposite also happens when navigating from a book with audio to a book without audio, the audio toolbar continues to show, although tapping it for a book without audio makes it disappear.
These behaviors can be confusing to users. The behavior does not occur when swiping back and forth by chapter. Version 12.2.2 release notes mentioned this bug (#1594) was fixed, but it still seems to occur.
This apk shows the issue. The app has the whole Bible text, but only audio for NT, Ruth, Esther, Ps, Prov, Jonah.