This happens from time to time: I wake up (too late) and see that the alarm has had no sound at all. Then when I check the phone, there's no ringtones, no alarms, no alerts, no message sounds, no sounds from the apps, no keyboard click sounds, absolutely nothing coming out of the speaker. Been late from work several times because of this. Yes, the mute switch is off. Yes, the volume is on. No, there's no dirt or dust in the headphone plug. I have never once even used the Do not disturb function. Only rebooting helps. But then this happens again in a while. The thing that really disturbs me that this is my THIRD (3rd) iPhone in a row that has the same problem. Since that's the case, I've been very hard trying to figure out - and hoping - that the problem is something that I keep doing. I just don't know what it is. Been using Nightstand app. Removed it now completely. That probably won't help either. Had my previous iPhone5 changed to a new one exactly 10 days ago for this very reason, and here we go again - the same bug is here. Not funny. I have been reading discussion boards a lot and it seems that this is a fairly common problem. However, nobody has come up with a solution. If someone figures that out, he/she's gonna make a lot of customers happy.