Suddenly, I'm having the same problem (ios 8.1.2):
Native alarm app: Was set for 6:00 a.m. Never went off...and when I woke up an hour late, it still showed as active (green).
Yacto Clock (third party app I used to use): Re-downloaded it, set for 6:00 a.m. this morning. Again...overslept. The app shows that it went off, but no sound was produced.
Each night, I listen to an audiobook, using the native timer to turn the audio book off in 15 minutes. Have been doing this for over a year, with the native alarm set---until iOS 8.1.2, everything worked fine. Audiobook would stop playing at the 15 minute mark...the next morning, the native alarm would go off on time, even with Do Not Disturb set.
Sure would love any insight folks have about this. Waking up an hour late is not. Good. At. All.
(My calendar alerts work fine.)