Hello Geraniumgrower,
A couple steps we’d recommend right off the bat for this situation:
Close and relaunch the app
Force the app to close. Then open things up, load one of your books and see if it works as expected.
Restart your device
Restart your iPhone then check to see if there is any change when listening to a book.
Check for updates
To see if updates are available for the Audible app, launch the App Store, tap your profile icon at the top right, then scroll down to see if updates are available. If updates aren't available for the app, update the software on your iPhone or iPad.
If the issue is persisting after that and is specific to the Audible app, Delete the app, then redownload it
Delete the app. When you do this, you might lose data that was stored in the app. Then redownload the app. Then test once more.
Thanks.