[DCP-o-matic] How to add 2nd subtitles to an existing DCP

Hello, I have a question about adding 2nd subtitles into an existing DCP. 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. Can you help with that? Thank you! Vojtěch Spěvák vojtech.spevak(a)seznam.cz