Aegisub

Aegisub – a subtitle program for the transcription

Here you can download the software:

https://aegisub.en.uptodown.com/mac

Aegisub is a free, cross-platform open source tool for creating and modifying subtitles.
Aegisub makes it quick and easy to time subtitles to audio, including a built-in real-time video preview.

The program does not seem to be intended for video and sound playback at the same time. They warn that it is not a media player.

So it might be preferable to transcribe with audio.

You can play the relevant track with subtitle and image separately.

When you are done, you have to save the subtitle file as srt-file.
Not with “save as” but with export and then choose SubRip (*. srt)

For better use

Maybe you better downsize the videofiles to a MP4-format with less quality, just for transcription purposes.
It depends on the computer’s internal RAM to handle the video’s well.

You produce a separate srt-file as transcription, so it can be linked to the original high quality video.

Useful is to also download the language dictionary from the site for the spelling checker.