If you don't see the bar under your name light up red, please check that the microphone image at the bottom left does not have a red cross through it. If it does, please click it to toggle mute on / off. If you still are having problems then please check to make sure that your microphone/headset is plugged into your computer and correctly configured.
If you have access to a different headset (or separate microphone and headphones) you can test those to eliminate your current headset as an issue.
When you join the lesson space you may also need to check the devices that the browser has selected, as the pre-selected devices may not be the ones that you wish to use.
Here are some links to help you:
Tips for fixing common sound problems Windows
Tutorial for configuring your headset on Windows 7
On a Windows PC you can test in the Windows Sound Recorder (normally in start menu > programs > accessories > entertainment) - if you can record your own voice and play it back in Windows Sound Recorder then the audio should also work in the lesson space.
If you can hear sounds from your PC in normal use but you cannot hear others in the lesson space, you may have your speakers setup for surround sound. To change it to stereo go to Start > Control Panel > Sound then click the 'Playback' tab. Right click your speakers and click 'Configure Speakers' Follow the wizard making sure that you set them to stereo and front left/right.
Chrome for Mac users
If you are on a Mac and using Chrome, you should restart your PC. Nine times out of ten this fixes the problem.