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See these articles for some troubleshooting steps that can help:
iPod touch (4th generation) and later: No audio or garbled audio on FaceTime or video recording
- Make sure that the microphone (located on the back next to the main camera) is close enough to be heard by the other party.
- Ensure that nothing is blocking the microphone, and if a protective case is used, try removing the device from the case.
- Try turning the iPod touch off and then on again.
If the issue persists after trying the steps above, try this one: