I received a dcp in OV in French with embedded subtitles in English. I would like to keep the English subtitles and add Czech subtitles.

This is how I tried to proceed (but it did not work): I created a new folder, then added files of existing DCP, then I added the new file with subtitles and place it so that it does not hide the original subtitles. Then I make the DCP. The problem is that when I play the new DCP the video is with both subtitles but there is no sound.