No sound on OIS

I use this app on my iPhone and there is no volume unless I plug in headphones. With the majority of the app being reading I dont want to have to plug in headphones every time there is a video. The app is great, although I hate reading the lessons are easy, but very time consuming. But again. No volume!

Terraria man 123 about Aceable – Driving School App, v1.26.1