Pinyin on hover!
Especially with the audio on hover not working, but even in general, I'd really like to be able to see a character's pinyin translation on hover. Sometimes I have trouble hearing subtle differences (z vs. c).
Z is /ts/ (unvoiced and unaspirated); Mandarin-speakers will hear /dz/ as /ts/ (owing to the lack of aspiration), but it isn't a voiced sound in Mandarin (similarly, English-speakers will tend to hear /ts/ as /dz/).
C is /tsʰ/ (unvoiced and quite heavily aspirated).
Voicing unaspirated consonants will allow the speaker to be understood, but these are not correct pronunciations.
I agree, the audio on hover being broken is particularly damaging to this course. I've actually taken a break from doing DuoLingo until they fix this and some of the other bugs, and I encourage others to do the same. If they saw a falloff in usage of the site following bugs like this, it might provide a stronger incentive for them to fix the bugs.