Click the 
icon on the toolbar.
This will open the Output Configuration
dialog box.
Click the Set button for the "ON-AIR" output device.
In the Select "ON AIR" Output Device dialog box you will notice a listing of your soundcard drivers.
If you are a Windows 2000/XP/Vista user you are best to use ASIO drivers. If you experience audio glitches with the ASIO drivers (or your soundcard does not support ASIO) you will need to use the WDM option. If you experience audio glitches with the WDM drivers you will need to either, calibrate the audio output device or use the Compat driver selection.
Click the OK button.
Make sure that the Enabled check box is enabled.
Close the Output Configuration dialog box.
Test your configuration.
Tip: If you notice any warbles and/or pops when using the WDM type drivers see How to calibrate your soundcard.
Note: For all other audio output related problems please see Audio output FAQ and Optimizing your machine for audio.
How to calibrate your soundcard
Optimizing your machine for audio