Hi Michelle,
This can be a bit tricky so no worries not finding it.
Option 1:
Go to hardware setup and click on one of your input channels and select configure. in the audio channel mapping change it from the default embedded setting to one of the AES/EBU settings only. I'm assuming the audio you are getting is audio embedded on the SDI input so changing this setting will cause the input to capture audio from one or more of the AES inputs where there should be no audio.
Option 2:
take your video material and drag it onto one of the audio tracks on your default scene director. Expand the track with the little black arrow at the far left of the scene director track. add a keyframe by hitting the red circle button. click on the keyframe and drag it as far down as it will go to duck the audio. This change will only apply to the one scene where as what I suggested above will apply globally.
Andrew
#XPression