Double check your media icon in the users list:
If you do not have an icon next to your name in the users list you have not joined audio.
Select the phone icon in the media bar to engage the audio join prompt.
If you have a headset icon next to your name in the user list you have joined Listen Only and do not have mic sharing abilities.
You can select the phone icon to leave listen only mode and select again to engage the audio join prompt, if you have a working mic, choose microphone when prompted and complete the audio test.
The user list will show a mic icon next to your name when joined.
If you have a mic icon next to your name and others still can't hear you:
Double check that your system audio in BigBlueButton is not muted; your mic icon will show a slash through it to indicate you are muted.
If you are not muted in BigBlueButton check that your computer system audio is not muted, and that your headset does not have a mute button on the cord that is engaged.
If you still can't share audio:
Be sure your device is set as the default device in your system settings.
Finally, try refreshing the browser and rejoining the audio.