Hi,
I have an openSUSE Leap 15.2. I haven’t use the Subtitles compose for years. And I open it today.
(Last time I used it, probably, was with the pre 15.x Leap versions).
My main issue is that I don’t hear sound from the videos and the video screen is black!
I can improve the situation with some of the back ends (like for example to have a video with the MPV player (what is that???) or have sound with GStream and alsa, but never both of them!