In 4.1, you can now change the playback speed while viewing presentations.
Step 1.) Locate your session on your organization's server. Click on the session.

Step 2.) Click on the drop box arrow on the lower right corner of your window.

Step 3.) Click on the playback speed you would like for this session.

After this step, playback will resume at the rate you chose.
Congratulations! You now know how to use the Variable Speed Playback feature!
If you do not have Silverlight 5 installed, you will see the following message when you try to view a session:

Click on the link highlighted in Red and follow the instructions on the new window in order to Install Silverlight 5. After installation, you may need to close and restart your browser in order for the change to take place.
Otherwise, download the latest version of Silverlight from http://www.silverlight.net/downloads and follow the instructions on the installation page.