If you've imported video file into Jitter and want to use it without its original sound or with the sound turned down, you can do this from the asset's properties panel. Here's how:
Select your video in the timeline
The property panel for your video will appear on the right side of the screen
Click on the volume value and change it to your preferred volume
Changing the volume to 0% mutes the audio track from your video for its duration of the animation.