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First make sure the silent switch of your iPhone is not turned on. It is on the left side, above the volume buttons. If you can see red on the switch, that means it is on. Switch it back.
If that didn't work, then 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. This will soft reset it.
Please let me know if that didn't work, so we can continue troubleshooting.