If I add a video to media and happen to have the subtitle file in the same directory then I can play the video from the media manager and the subtitles will display. I can add the item to a service and it will still play the video with subtitles. Once I save the service the subtitles do not play. I assume this is because when I play from the saved service it play the video file from the servicemanager folder not from its original location.
The only workaround I have found is to leave the video out of the saved service and load it into the service at the last minute and run the service from there without saving. This is prone to error because I will often find some other change need for the service and then hit save without thinking and when the video plays later I discover missing subtitles.
Ideally the media item would allow you to add additional files so there is video.mp4 and video.srt in the item with the first one being the one that gets played.