I think there's an alarm glitch with the latest beta version of iOS 9.3
Last night, I set a new alarm for 0830 and set it to play a particular song. It showed the correct song selected, but started previewing something else entirely (Alphaville "Forever Young" from my Napoleon Dynamite soundtrack). I have iTunes Match, so most of my music is in the cloud. A lot of tracks/albums were manually downloaded from iTM to my device, but it should also intelligently cache and should work like everything is on my device.
I don't normally have this, but I had a white noise app (Relax M) playing last night.
I woke up and was using my phone before the alarm went off. My phone froze for ~10 seconds. It was vibrating, but frozen on the web page I was looking at and eventually showed the Snooze/OK dialog after 10 seconds. I pressed Snooze, then killed the white noise app. The alarm activated again 9 minutes later and still only vibrated (no sound). I snoozed the alarm again and kept using my phone, this time with the ringer switch on. Still no sound when the alarm activated.
Could definitely screw someone up if they depend on the alarm to get up for work. I know it's beta software, but Apple has a history of glitches affecting the alarm.