The audio-only files do not play with the plug-in in iOS. I have tested this with m4a, mp3 and wav file formats using these files:

mp3 file
m4a file
wav file

These files play fine on Android and in Web.

I have simply used the example that comes with the plug-in to replicate the issue. Simply replace

    _controller = VideoPlayerController.network(
      'https://flutter.github.io/assets-for-api-docs/assets/videos/bee.mp4',
      closedCaptionFile: _loadCaptions(),
      videoPlayerOptions: VideoPlayerOptions(mixWithOthers: true),
    );

with

    _controller = VideoPlayerController.network(<url_of_any_of_the_above_files>);

The controller's initialize() method does not complete and does not return any error. The issue can be replicated on the iOS simulator (iOS 14.5) and a physical device (iOS 14.7).