Still investigating (can't see on actual device yet; only simulator since we don't have a test device in the office with 9.3):

Try and tap "listen" for an audiobook. Effect: App Crashes.

It's about 5-10% of our userbase. Since we didn't press "submit" to Apple yet I think it's best to at least address the crash even if I can't fix the issue for those users. (edited)

Fix added on Friday, see comments below for acceptance criteria