Not all videos and audios available on nPlayer can be played on Chromecast but only those that meet the standard of Chromecast can be. Therefore, the file format and codec that are not supported by Chromecast cannot be played.
For subtitles, Chromecast does not support embedded subtitles and graphic subtitles format(ASS, SSA etc) currently. For this reason, subtitles can be played on the device when playing embedded subtitles and graphic subtitles, but they are not displayed when casting with Chromecast. If you want to play embedded subtitles and graphic subtitles on Chromecast, you should contact Google. Please check your subtitles format.