Select the default audio playback device from the pull-down list. If a sound device was installed or plugged in after the options dialog was opened, you will need to re-open the dialog to refresh the list of devices.
To activate the audio ducking for a sound clip in the sequence, right-click on the waveform and check on "Background Sound" otpion.
Ducking Attenuation: The amount of attenuation applied when ducking
Ducking fade in length: The length of time over which the ducking is activated
Ducking fade out length: The length of time over which the ducking is released
Ducking Threshold: Ducking is activated when the volume of main sound hits this level