YouTube allows you to easily create the SRT files needed to better engage with your viewers during silent autoplay ...

Roll your own subtitles An .srt file is simply a text file formatted in a specific way. You can create your own subtitles and add them to your movies, as described above.

Create A Transcript You now need a text file of the actual words to your audio/video, and in the proper format (SubRip, aka .srt).