Alarm only vibrates - randomly

So--- I found this odd the first time it happened, but it has happened at least two more times randomly so I thought I would see if anyone else has had this happen.

Right before I go to bed, I click on the alarms I have stored-- I usually set 4 within 10 minutes or so of each other. I make sure everything is set for AM, volume is turned all the way up, and that the vibrate switch on the side of the phone is off.

Usually-- everything goes off as planned with the most annoying sounds from the iphone available ringer sounds (my phone is original and not jailbroken). I plug the phone into the charger and go to sleep.

But at least three times since July-- the ringers have not actually sounded and the only thing that sounds is vibrate.

I have even restored my phone a couple of times and this has still happened-- luckily the vibrate noises wakes me up but it concerns me that I cant fully rely on the phone to wake me up since this vibrate only has happened a couple of times.

Btw-- the last time this happened when I unplugged the phone-- the next alarm went off with the alarm ringer--- another time the phone was unplugged when this happened and the audible ringer went off when I plugged the phone in-- so there does not seem to be a correlation with being plugged in or not. Thanks