If you are using a Mac and Google Chrome, please see this article.
The first thing to check - have you turned your volume up?
If the red bar moves under your picture in the lesson space when you talk, then you are broadcasting sound and the issue is usually at the other user's end.
If the red bar is not there or does not move, try a full restart of the computer, this will solve any hardware issues.
If that does not work, changing the headset/headphones that you are using can often solve the problem.
If this does not work, you can try the steps below.
Mostly, more complex problems are caused by the microphone not working or the sensitivity of the microphone being turned down too low.
You can follow these steps to check if this is the case:
1. Open 'Control Panel' > 'Sound' > 'Recording Tab'
You should be able to see at least one microphone there. If you can see a microphone there, when you speak the microphone level indicator to the right of the microphone should move. If it does not move, this indicates that your microphone is not working properly.
2. If the indicator does not move, you should right click the microphone and click on 'Properties' > 'Levels'
The level that you want to check is the microphone (not the boost). This should be no lower than 70%.
3. If the volume is lower than this (image above) please move it up.
4. If you have done the above and the microphone does not register (step 2) there is a chance that the microphone is broken.
5. A final (and unlikely) issue could be that you have configured surround sound on your computer.
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.