I have a difficulty with auditory processing. It’s usually just a matter of concentrating more and/or listening a second time, but it would be way more convenient if I could just read along with the voice over. There are many others who have or could have these issues, and similarly some buyers of the course could be hard of hearing and would find it quite difficult to watch without subtitles. For this reason I think it would help immensely if you add subtitles to the course. You don’t need to burn the subtitles into the video frames, with no option for the viewer of removing them; MP4 files have something very much like the subtitle track of a DVD or Blu-ray (or indeed a show on streaming), where you can embed either pre-rendered bitmap subtitles akin to what DVDs and Blu-rays tend to have, or encode the subtitles directly as text, to be rendered at runtime by the video device or software. If you rather keep the files separate, then if you place a subtitle file in the same directory as the video ones and name it exactly the same as its corresponding video file, it will be picked up by most video players as the default subtitle file, and will be loaded along with the video.
I can write subtitles that comply with professional subtitling standards, and if you prefer you can hire me to author the subtitles. Or you can hire others, or just not do it, that’s your call.