The following information is from Google's help page which can be found here.
If you have feedback or hear background noise while in a video meeting, you might want to mute other people’s microphones. For meetings organized through a personal account, only the meeting moderator can mute other participants.
To mute others, next to the person’s thumbnail, point to Volume Mute .
Tip: For privacy reasons, you cannot unmute another person. Ask the participant to unmute their audio. To mute or unmute yourself, at the bottom of the video window click Mute .Phone participants can press *6 to unmute their phone. Any participant can mute. People who dial-in by phone can also mute.