There are two ways for your app to notify the user while the app is in the background: push notifications and local (or scheduled) notifications. This tutorial explains how to create local notifications. Local notifications are simple to use but are also quite limited. While your app is running...
Thank you DonManfred for your reply, I think it's my only option for iOS, maybe I'll just take a sample of the 30-second audio file and then set it as a custom sound for the notification.