This is a conversation between my son and I this evening.

hahahaha, that just happened to me today. My phone would vibrate but no alert sound. I checked and it was all set properly in the utilities. Then for some reason I looked at the button on the side, and sure enough, the little red line was showing that it was off, lol. I never did actually flip it off, but it must have gotten pushed to off when I was trying to get it out of the case to clean it.