I found that iphone apps do not run in the background except for certain cases like voip, music player and GPS apps. I found this app in the app store, it does not belong to any of these categories, runs in the background and provides sound alerts in the background.
How is this possible?
Apps like this usually do workarounds based on things you mentioned:
UPDATE: From Info.plist of this aforementioned app:
Using Xcode documentation:
Therefore this app uses first approach (i.e. it mimics music player) to run in the background.