When you start a meeting you can enable or disable voice chat. When someone joins your meeting this will set their audio to default ON or default OFF.
If you are the host of the meeting, you will be able to mute any participant. If you are not the host, you can only mute yourself.
Participants with audio enabled will have a solid white microphone next to their name.
Hover over your name and click "mute" to mute yourself:
Participants who are muted will have a grayed out microphone next to their name:
Using the VR Menu is a similar process.
Aim at a name in the Meetings tab of the VR Menu and you will see the Meetings tools appear. Click Mute or Unmute using the trigger of your VR controller to select it.