iPhone alarm doesn't go off sometimes! At times, the alarm in my iPhone 4S sometimes doesn't go off as set. Or actually, it goes off and I see the corresponding UI on the screen but there is NO sound.

This happens very randomly, may be once in ten times but is obviously not acceptable in an alarm application.

This mostly happens when the phone is in silent mode (which is still NOT expected behavior I am sure) but has also happened a couple times when the phone was non-silent.

How do I fix this?
This could have to do with a software issue on the phone. Try backing up your phone via iCloud or computer, then wipe and restore it as new. If it doesn’t fix the problem, take your device to Apple as it could be an internal hardware issue.