Hmm. Somehow I never saw replies to this original post.
I think what I’ve been encountering are two issues, one of which is resolved above:
Not every word in the course has audio. (Understood)
For listening tests, sometimes the audio buttons turn red and have no sound, leaving only the correct answer with actual audio available.
The second issue probably stems from the first, and makes listening reviews fairly hit or miss in how useful they are in this particular course. I suppose the 2 “wrong” audio clips are being selected from the full course vocab rather than only the vocab that has audio.
FWIW, most of my courses are official Memrise courses and don’t have this problem. I’ll keep an eye out in other community-generated courses and see if a similar issue persists.