This has to do with missing ringtones. I have a ZTE maven 2 model 851. My phone just upgraded itself to Android 7.1.1 All of a sudden (around the time of the upgrade) most of my sound appeared to go off, but the phone still made its startup sound on power up and a beep when I hit a key. But I had no ringtone when someone would call in, and when I tried to select a new one there was no playing of the ringtone. After a long time puzzling over this I finally went into SETTINGS - SOUND, and noticed DOLBY AUDIO sound enhancement was ON. I had not set this, and had no idea how it got set, so I set it OFF. After this my ringtones came back and everything appears normal, at least as normal for this phone. Hope this helps someone.