What shall I do if there is only sound output and the screens show nothing?
The connections at both the Apple device and screens are secure.
The video you are playing is not of HD (High Definition). The product supports only Composite Video (SD video) output. Therefore, HD video cannot be displayed on the screens. If you have HD video, convert it to SD, or download the video in SD version.
The file you are playing is not a music file (such as MP3 file). In this case there would be no video display on the screen.