I just got a hand-me-down iPhone 4, and there is one issue that is driving me absolutely nuts. The sound is intermittent. I.e., go to Settings, then Sounds, then Ringtone, if you select a ringtone the tone plays fine. Then you go to Text Tone and select one - all I get is a vibration. Also, when on Control Center, there is no volume slider as shown in this screenshot. I'm completely baffled. After I unplug this phone from my car stereo the sound works... For about a minute before it goes out again. I'm obviously not always around my stereo. Please help?