Have you checked the following:
1. Have you got the latest version of the UE app installed in your Ipad and Iphone?
2. Is your UE Boom firmware up to date? (Use the app to find out).
The following are all reasons why it may play the backup sound.
Is your app still running? If it is switched off the UE may default to the backup sound.
Your phone may be out of range of your UE BOOM, make sure the two devices are within 50 feet of each other.
The Bluetooth connection is Off on your phone or your phone is in Airplane mode, ensure that the Bluetooth connection is On.
Your phone may be off (or out of battery). Ensure that your phone is On (your alarm will still play if your phone is Locked)