You may need to turn up the volume.  This can be done during the conference by selecting the Video Settings button.

 

Then select the microphone tab to make adjustments.

 

If this does not work there may be adjustments made by your computer outside the conference.  For example, Windows 7 is set to automatically reduce a computer's volume when it recognizes a conference taking place.  Follow these instructions to fix the issue:

1. Click the Start icon in the bottom left-hand corner of your screen. Next open the Control Panel.
2. Click on Hardware and Sound:
   
3. Click on Sound:
   
4. Select the Communications tab:
   
5. Select the option Do nothing. Click Apply and OK: