I should add that I have a similar problem.
Learning Hebrew, using the template from Fluent Forever.
Everything worked perfectly, for many moons. Then, some time over the summer, I would add a new card, and add an mp3, but when using the deck, no audio plays, and there is no play button. E.g. even though the Audio was added the normal way (dragging file from Windows Explorer onto the right edit in the card), it is as if the audio is not there at all.
Just now, I browsed the deck and went to a card that had audio that was not playing, I cleared the file name from the card, moved off the card, moved back on, and dragged an mp3 onto the card. Annnnnddddd, it still does not work.
Older cards, that I made before the problem started, play audio fine.
I am running 2.1.33 on Windows (where I just did sequence above), and whatever the current Android version.
The problem is visible on both windows and phone.
Please give me steps to chase. (I am technical, fwiw)