The FFmpeg media plugin is a primary media backend that must be delivered and loaded properly. Running the examples without extra logging may not reveal which media plugin has been loaded.

To check that the FFmpeg plugin has been loaded, one of the following classes should be used in the tested example: QMediaPlayer, QMediaRecorder, QScreenCapture, QWindowCapture, QCamera, QAudioDecoder, QMediaCaptureSession.
The FFmpeg plugin writes development logs with the logging category qt.multimedia.ffmpeg.XXX, checking the logs can be an indicator that the plugin has been loaded.

To turn on the logs, you should set the environment variable QT_LOGGING_RULES:

export QT_LOGGING_RULES="*.ffmpeg.*=true"