In addition to the volume controls on your external speakers, there are additional volume levels that can be adjusted on your computer.

Flash Option
If you are using the Flash option to listen to the webcast, you can adjust the volume of just the webcast audio from within the media frame. Within the media controls, you will see a volume control to the right of the speaker icon. You can decrease or increase the volume level here from 0% to 100%. This volume level is not part of the media controls for Windows Media and Real Media option (where you can only mute and unmute the audio).

Master Volume / System Volume