This is probably because his/her microphone volume is low.
You can try and solve this by asking your friend to increase his/her Microphone volume level.
To do this one, follow these steps:
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Click on Start > Settings > Control Panel.
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Double click on Sound and Audio Devices.
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Select the Audio tab and click on the Volume button in the Sound recording section.
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Move the microphone volume slider up, until you hear each other better.
