Sound issues on iPhone 4S

There is no sound coming out of the speaker other than ringtones. No keyboard clicks, music, or sound from videos. I plugged up my earphones & it began working for a while but then it goes away but the music play button is still on the screen. I only use the apple charger that came with the phone. The only time it works fine is if I'm playing music in the car & the USB is connected. I have restarted the phone numerous times but it still messes up. There is no dirt or anything in the dock connector or earphone jack.