You haven't inadvertently turned on the mute button have you, it's the little switch just above the sound control.
Also look at the control which is found when you bring up your multi task bar. Do the 4 finger swipe up or double press the home button and you will see your multi task bar along the bottom, swipe right as far as you can go and you will see a volume control slide here. Also another place in Settings - General - Sounds. Let us know how you go.
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