Using MULTIMIX 4 USB FX how can I control the volume of the USB output without using the Main Volume Control Knob

I am outputting usb audio and a microphone simultaneously during a live show, (I'm running PowerPoint from a laptop). I would like to know how I can fade out the USB audio individually without using the main volume control knob and fading out the microphone...

I am using myAlesis MULTIMIX 4 USB FX.

Any help would be appreciated.
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