I am having a similar (serious) issue. Samsung Galaxy S5 (AT&T, Android 4.4.4) messaging app is not rendering sound. When messaging app is closed I receive an initial sound notification for received text message. When chat window is open and texts are received there is no sound with each text (and if window stays open while you are doing something else you have no sound notification of text received within that thread). I've rechecked all sound applications / volume levels, etc. In messaging app settings there is no setting for "Sound," by the way (though I seem to recall one BEFORE the AT&T update to Android 4.4.4 a few weeks ago). I have also checked SD card, no problem there. I have also rebooted several times, soft booted, turned messaging app on / off, deleted cache and data, etc. Nothing works. Finally, I did a complete factory reset yesterday, still no sound for additional texts when the chat window is open. Any suggestions much appreciated. Thanks.