Sound stopped working and unable to use offline lessons after paying for Plus
I bought Plus just before a long flight where I wanted to be able to do lessons offline, and everything worked fine for two or three lessons, after which the sound stopped working. I can hear the sound effects when I get an answer right or wrong, but no audio will play within the lessons themselves (though when I tap individual words, I'm usually able to get the last word in a sentence to play very faintly). Also, even though I downloaded most of my tree for offline use, I'm only able to access around three or four skills when I'm offline.
So far I've tried:
1. Looking for updates (I use the iPhone app).
2. Quitting the app.
3. Turning my phone off and then back on.
4. Trying lessons in other languages.
I'm just annoyed because I paid for Plus specifically for use during a single flight, and the app has instead become unusable. Does anyone have any solutions? I'm happy to give up my Plus subscription if it would help since I no longer need it!