I am currently on CM11-M3, and have had the problem since 10.2. No matter what ringtone I select it will not play during an incoming call.
I have been testing with wheresmycellphone.com. Ehen a call comes in the phone will vibrate once and very rarely the ringtone will play for a fraction of a second, but most of the time there is no sound. I have noticed that the mute icon (speaker with a line through it) flashes in the notification area briefly the second the phone starts ringing.