Hello!
So, I recently opened a new lesson (one I’d never opened before). I was able to convert all blue words to yellow without issue, but the audio does not load. In fact, it says it has loaded “124%” but never resolves.
This is true on my iPad, I have not checked a PC.
As a side note, when converting blue words to known, they do not appear to sync with the Known words count on the site at the moment. If you turn them yellow and then set them as Known, it works!
The lesson was Part 3 of the I Am A Cat by Natsume Soseki course for Japanese.
Thank you! If you need any further info, let me know 
Ben.