Ще is used as
1) yet (Я ще не їла - I haven't eaten yet)
2) still (Ти ще хочеш їсти? - Do you still want to eat?
3) more (Я хочу ще один - I want one more)
4) even (Цей гарний, цей кращий, а цей ще кращий - This one is good, this one is better, and this one is even better)
5) ... I don't even know we use it for so many things
Many people have commented and reported about this.
It happens with the exercises that had no audio before, and recently newly recorded audio was added to them. For some reason there's a bug, audio doesn't play during listening exercises. Works using the browser but not the app (on Android I think).
I've notified the people working on this feature, they have found the cause of the bug. Maybe they haven't fixed it properly yet, or there's another bug :') Will notify them about your comment! Thanks!