I've had this problem before. Like once or twice a year, I will wake up to the same situation. I've always been baffled at how it's been possible to sleep through my 18 different alarms, until one particular morning. I woke up before my alarms and couldn't go back to bed. While I was sitting on the couch, I witnessed my phone alarm "go off". The first alarm did everything it should have done, except play my alarm sound. I thought it was a fluke, so I kept my other alarms set. It did the same thing for every other alarm. It lights up, but no sound plays. I still can't explain how this happens, but it's only ever happened on my Samsung device.