iPhone 4/5 alarm clock problem

Hello everyone!

I wanted to ask for an advise here, as nowhere else I found a solution.

I had an iPhone 4 with the same problem and that it moved to iPhone 5 I purchased recently.

The problem:

I have 3 alarms set with 15 mins gap. I had no issues with time zones ever. When the time comes the alarm goes off, but without any sound. I just wake up late for work after an hour of deep sleep, slide to unlock and see a message asking if I need to tourn alarm off or repeat after some time. No sound is played.

That means that alarm went off in time, the message was shown on the screen, but the sound wasn't played.

That happens with no reason and randomly. And I'm pretty sure I'm not a sleep walker.

Can anyone tell me what's happening with my both iPhones?