I Have the EXACT same problem. I went on YouTube and I came across this channel...It solve my problem!!! Guaranteed!! It worked on any iPhone As well. Basically there is a switch just above your volume and up button on the side. All you need to do is to switch up where it does not show the RED color. And that will solve iT.
Right now we are all having a problem where our iPhone goes silence when there is incoming texts in our notification locked screen or when we don't hear a locked screen sound when we lock our phone is because that switch above the volume up/down button is switch to red where it mute text tones, etc. I hope that will helped. If not, check sound under setting as well as do not disturb under setting the manual should be off not on.
http://youtu.be/_gFgdxuxwlQ