Just flick the silent switch.
I know from experience, I learned the hard way that an alarm at 6:00am won't do anything in regards to waking me up if the silent switch is still flicked, which happened from time to time. All you need is forgetting to put it back to "sounds on".
PS: It'd be nice if Apple allowed us to have some alarms marked as "important" or something so that they ignore the silent switch.
Anything to make iOS a rock solid alarm clock.
Glassed Silver:mac