Omigosh, duh!  Thank you!  I work outside of the country and was about to panic about not being able to get back before the 3 month warranty ends to have the phone checked.  (I don't trust the Mac "certified" dealers here).

I don't know if I'm having the exact same problem as you guys, but recently my iphone 4s had stopped ringing on Lock Screen once I upgraded to the 8.0 OS or whatever. The solution for me was this weird setting they added in General Settings called Do Not Disturb. it defaults to basically never ringing or sending an alert at all on lock screen. So, what I did was only set it to not ring between 3am- 4am. And now it rings! Ridiculous. I hope that helps!

iPhone 6

this worked for me

Passcode /Touch ID enabled :

Will alert if Notifications>Allow Calls is set to Everyone; Will not Alert if set to Favorites( even if receiving call from a Favorite)

With alerts set to Favorites I was not hearing text sounds, even when changing them to a new tone in settings. Switching the option to Everyone and then I started hearing the sounds played as I changed them in settings.  There could be more than one way to solve this problem since others are offering other solutions.

I have had this same problem as well. Checked all the normal sound, message, notification settings and everything was okay. Finally found the problem. Under "Do Not Disturb" in "Settings", the "Manual" setting was set to "On". That blocks all calls and alerts when the phone is locked. I did not turn that switch on, but it was on. I did connect to a bluetooth speaker and microphone device yesterday for a conference call. I noticed later in these responses that connecting to such a device may cause problems. So now I wonder if that didn't get automatically turned on when I was using the bluetooth but didn't get set back to off when I disconnected. One sign that might be the problem is if your half-moon icon comes on in lock screen. That means your DND is on.

At any rate... it's all working for me again. Hope that helps.

The solution is to "flip up on the open screen from the bottom and you will see airplane mode, WIFI,etc and towards the bottom on the right side you will see the last Bluetooth equipment that you have used. Change that to AIRPLAY and viola, you will now receive sound on your iPhone!

Like most others here I've also had this issue (with my previous 4S too)

My solution was to clean the lock switch that turns your phone on/off silent (the gray/orange switch on the side) I just did that with the edge of a metal ruler.

I found that clicking the switch wasn't 'switching' properly, the ringer symbol wasn't coming on or off, after a little scrap in there with the ruler, all is well again.

Good luck!

Thanks for this. This is where my problem was. It was driving me nuts.

I don't know about the rest of you, but I figured this out and feel like a moron now as I spent 3 days trying everything to figure this out.  When I pushed up to bring up the apps in that lower menu I had "Do Not Disturb", the little half moon on.  I must have accidentally hit this without realizing it.  I hope this helps someone else!

Hope this helps someone. I tried everything people listed and nothing helped. I played with it until I did the following. Go into settings, then do not disturb, then  I went into allow calls from and set to everyone. Everything is now perfect. Hope this helped.

Hope this helps someone. I tried everything people listed and nothing helped. I played with it until I did the following. Go into settings, then do not disturb, then  I went into allow calls from and set to everyone. Everything is now perfect. Hope this helped.

OMgosh, DUH me!  Did y'all know there's a little tiny switch above the volume control on the side of the iPhone 6!

Turn Silent Mode On/Off - Apple® iPhone® 6 / 6 Plus

[Device-Specific Instructions]

From the top left edge of the device, flip the Ringer switch to turn on (switch in red position) or off .
The silent mode/ringer notification briefly appears on screen.

From Osxdaily.

It works.

"With almost certain guarantee, the reason an iPhone is not ringing or making alert sounds apparently out of the blue is because of the Do Not Disturb feature. Do Not Disturb is absolutely fantastic when you use it properly and set a schedule for it, but if you’re not versed in how it works, it can be accidentally toggled on and lead to all sorts of confusion and missed calls or seemingly ignored text messages."

I just updated my phone where I received that annoying Apple Watch app that I did not ask to be downloaded. However, with this latest update, it changed something where I no longer get tones for text messages when the screen is locked and I have checked the "Do Not Disturb" feature in both the settings and on my phone.  Even with going back and resetting the tones to my text alerts, I still get no sound for text messages when the screen is locked, where as before the update, there was no problem with alert sounds when the screen was locked.    Anyone have this issue?

I've had and I'm still having this issue. For me it has nothing to do with "Do Not Disturb". If my iPhone has connected to any bluetooth device at all I have to turn Bluetooth off after I disconnect from the device in order for ringtones or notification sounds to return to the iPhone's speakers. It's a huge bug with iOS 8 that I assumed would've been fixed with 8.2, but it still persists and it's the same with my wife's iPhone. We both use bluetooth to connect to our car and have to remember to turn it on every time we get in the car and turn it off every time we get out. If we forget to turn it off we typically miss a bunch of phone calls and messages because our phone that should be notifying us is still trying to play the sounds through the now disconnected Bluetooth device. Same goes for all our Bluetooth speakers, so it's not just an issue with one particular Bluetooth device. If you are having this issue and turning Bluetooth off fixes it please report it to Apple. It's the only way we'll get enough traction with this bug for them to fix it, because it's a ridiculous bug for such a mainstream and expensive piece of hardware to have.

try switch at top left on the side.

WOrks great when this unlabeled switch is turned on.

I accidentally hit it when removing protective cover.