Hi Patrick & Elton,
Thank you for noting this problem. I have also experienced the app crashing when playing mp3 files or videos in the quiz module, both before and after the recent Moodle Mobile update (3.5). The problem appears to be limited to iPhones fortunately, but it has affected multiple iPhone users with different phone models and obviously varying amounts of free space, perhaps different iOS versions, etc. I've already voted for the tracker issue and will follow it.
I would like to add that the issue is not restricted to ios. Android has the same problem.
About 100 users with me are facing this problem on Android 5 to the latest 7.
Thanks Elton and everyone else for noting the problem and opening a defect.