The video signal sound is output as analog stereo signal by some video input cards. Currently, there are two ways of further sound editing:
You supply it to the line input of your sound card as recommended by the graphics card manufacturer, so that you can add it to the sound track of your show or mute it, etc. via the Windows Mixer.
For an installation, you should provide for the live video sound to be supplied via a serially controllable mixer. Such a mixer allows sound control via the corresponding command markers in the timeline.
See also