Network signal is not good
Maybe the local network signal is not good. Change a WeChat contact to make a video call, or switch the network connected to the mobile phone.
Clear the WeChat data or reinstall it.
Go to Settings-More Settings-Apply-All-Find WeChat to clear the data, or uninstall WeChat to download and install again.
System update
Restart the phone and go to Settings-More Settings-System Update-Download and install the latest system version.
Microphone settings
Many people like to turn off the "Mute" option in the "Microphone" column of the volume control panel and turn the microphone volume to the maximum before using the microphone for video chat.
Headphone settings
It is suggested that you choose the "mute" option when using the microphone, which can eliminate the noise and echo in the earphone and make the voice effect better.