Did you go into "Settings" and then "Sound" to see if all is set to the same things? Ringtone, etc. And make sure the ringtone is still on the device.
Also check the volume button. Yes I know it seems like common sense to check that, but so many people do not.
Next I would make sure that it wasn't put on silent (it happens). And, if you installed any new apps just before the problem started, I would try starting up in "Safe Mode" and see if the ringtones work. If they do, then you have an app, malware, virus, etc., that is interfering with the sounds. You need to find and remove it.
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.
Should none of this work, take the phone back and have them exchange it as there is a problem with it.