I have experienced this persistently between sessions for 2 weeks with no other known issues, when using Modern Hebrew on Android.
There is one view of the quiz screen where a single large audio playback button at the top of the screen followed by a list of answer choices. The playback is not automatic and the button is always nonresponsive. Audio works in all other quiz screens.
I have experienced a very similar bug on my Android app version of Memrise. In the Listening Skills quiz, when you get shown a word and have to select the correct one of 3 audio buttons, one will be greyed out, and the others will not be the answer. This has happened not consistently every time, but a few times for this specific type of question. Forces me to stop audio questions, while I find those very useful normally.