my iphone doesn't ring and notification sounds do not work

mine is doing the same thing (4s). No it is not on silent. And the missing sounds are intermintent. Definitely behaving like a bug. And it is a pretty significant one when this is a work device and I'm waiting on calls, messages, etc.

Is the silent switch on the side of the iphone on?

you either have it on silent mode or you have notifications turned off

go on settings, check out notifications, if "do not disturb" is on, turn it off, it should fix the issue.

if that does not remedy the issue try this

go on 'sounds', and make sure all the on/off sliders are ON