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It is possible that you've enabled the silent switch on the iPhone. On the left side above volume buttons, you will see a small switch. Do you see red on it ? If so, switch it back and this should fix the problem.
If that didn't work, then we can reset your iPhone and see if it works or not. Press and hold down the Power button and Home button at the same time until it shows the Apple logo. When you see the Apple logo, release the buttons and let it boot.
Note : Power button is the button you use to turn off your iPhone or its screen. Home button is the round button at front bottom. (Make sure you know the passcode of iPhone before resetting as it will ask for the passcode after restart)
Please let me know if that didn't work, so we can continue troubleshooting.
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