I spent an hour today trying to troubleshoot why the Migaku Player was not creating cards when I imported an MP3 file into the player with accompanying subs in SRT format. I fixed the problem by creating a video file of the MP3 that shows a static image.
The issue here is that there is no indication in the UI that this will not work. The player plays the audio with the subs as one would expect giving the user the impression that card creation will succeed. The player should at the very least warn the user when importing that MP3s are not supported or display an error when card creation fails. No unhandled exception was shown in the developer tools menu either.
Obviously the better option here is that this should be supported as there are a lot of audio only podcasts that can be used to generate SRT files and then loaded into Migaku. One should not need to generate a video file from the audio.