i don't know out it is just my phone, but i have never been able to hear any of the sentences.
I have the same problem. I can't hear them on my phone - Android, but I can hear them on my computer. It's not just you. It's like you can't see the learning tips on the phone app and they are really useful so it's worth trying to use both systems.
Thats where people say there are lessons I'm not seeing! Im exclusively mobile and could NOT figure out what they were talking about