Answer
In multi-screen mode audio for the camera image selected by a red frame is output.
1-screen mode : Audio for that screen is output.
Multi-screen mode : Audio for the camera image selected by a red frame is output.
Full-screen mode : Audio for the top left camera image is output.
You can also fix the camera CH that outputs audio by "Audio output camera settings" of "Advanced settings" of monitor settings.
No audio is output during the sequence.
No audio during particular playback. (such as high-speed playback)