Mine is iPhone 4S (warranty is void). I'am having the same problem. Sometimes back to normal (after idle for a while), but not last. Turning off the phone raises back the issue.
- On volume up/down button clicked: Ringer overlay is shown but not bars
- No lock/unlock sound
- No notification sound
- It's ringing when receive phone call or set volume from Sound setting
- After turned off, iPhone won't turn on unless plugged to charger
I've done these:
- Cleaned up charging port
- Dryed the iPhone (hairdryer; below the sunlight) - just in case
- Reset all settings
I haven't restore the phone since I'm not sure if messages, mails, WhatsApp, LINE (etc) histories will back after synced from backup.
I read that deleting com.apple.iapd.plist (Root>System>Library>LaunchDaemons) can fix this, but it needs jailbraking (iFile/iExplorer) which I don't want to. Furthermore, "Deleting the 'com.apple.iapd.plist' may effect the functioning of hardware accessories that go with various apps, such as FM transmitters and Nike accessories, etc." —wisso (https://discussions.apple.com/thread/3790682?start=45&tstart=0)
I'm not sure if some apps crash with latest iOS update and cause this.
I'm still looking for the right solution.