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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:

1. Click on Start > Settings > Control Panel.
   
   
2. Double click on Sound and Audio Devices.
   
   
3. Select the Audio tab and click on the Volume button in the Sound recording section.
   
   
4. Move the microphone volume slider up, until you hear each other better.