There is a microphone symbol located on the meeting toolbar. The microphone (audio) may be active as soon as you join the meeting. However, if there is a red line through the microphone symbol, the mic is muted. Select the icon once to remove the mute command and activate the audio broadcast.
The leader of the meeting may mute microphones and stop video (webcam) broadcasts during the meeting. In this case, use the Chat tool to request use of the microphone from the host of the meeting.