Apple doesn't accept submission of the app into the store using just_audio_background package for flutter

I followed the instructions on how to setup an app to use background audio from the just_audio_background package (https://pub.dev/packages/just_audio_background).

Unfortunately in the next app submission to the store, apple declined it with the statement:

In text:

Guideline 2.5.4 - Performance - Software Requirements


Your app declares support for audio in the UIBackgroundModes key in your Info.plist, but we were unable to play any audible content when the app was running in the background.


Next Steps


The audio key is intended for use by apps that provide audible content to the user while in the background, such as music player or streaming audio apps. Please revise your app to provide audible content to the user while the app is in the background or remove the "audio" setting from the UIBackgroundModes key.

The background functionality works for both staging and production apps from TestFlight, I can put the app into background or lock the screen and keep listening the audio, play/pause from the Notification Center, but still not accepted by apple.

What is missing from the README file of the package that should be added to get accepted by apple?

This is the screenshot of the player in the Notification Center:

I setup everything as explained in the package readme, they work on TestFlight as expected, but were rejected by apple as described in the question.

So, we just needed to create a video of the background audio functionality working and send it to apple as a reply in the apple review, so the reviewer reconsidered and approved the build to release. Basically we had to show that our app actually has audible content and that it works in background.