I have iphone 5S. My alerts have quit sounding when my phone is in locked mode.
"Do not disturb is not on, mute is not on, volume is turned up. In "Do not disturb" setting the only choice under "Silence" at bottom of screen is "Always" or "only while iPhone is locked". No silence "off".
I have rebooted, reset sounds, etc. it shows alerts on screen but not sounds. When not in locked sounds work fine. My husband also has iPhone 5S and his works fine! All our settings look the same. Thanks!