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I have a couple of recommendations for you here. First turn the mute on, then back off and retest the issue.
If the issue persists I would follow these troubleshooting steps:
- Verify that there is nothing plugged in to the headset jack, including headsets, headphones, or adapters.
- Make sure the Ring/Silent switch is not switched to silent.
- While on the Home screen, adjust the volume buttons. If you see the icon below, indicating that headphones are attached, there may be debris or an object lodged in the headset jack:
- Check the headset jack. If there is an object lodged in the headset jack that is not easily removed, have the iPhone serviced to remove object.
- For Original iPhone: If there is light debris, such as lint, in the headset jack, try connecting a pair of headphones to the headset jack and then remove the headphones. Repeat this several times to remove the debris.
- If you have installed a protective film on the display, either ensure that the receiver is not covered or remove the film completely.
- Check the receiver mesh (which is on the top front of the device, above the display). If it appears blocked, use a clean, small, dry, soft-bristled brush to carefully and gently brush away any debris.
- If you have paired with a Bluetooth headset that is nearby, either use the headset or turn Bluetooth off (choose Settings > General > Bluetooth).
- Restart the iPhone.
- If restarting the iPhone doesn't resolve the issue, try restoring the device.
- If the issue persists, go to the Service Answer Center - iPhone for information on service.
From: iPhone: Can't hear through the receiver or speakers