STEPS TO REPRODUCE
1. Launch iClone 6
2. Apply project with a prop called DefImagePlane
3. Open Timeline, expand the Video track of DefImagePlane
4. Go to any frame of "Image_PlaneOpacity Volume" track and double-click it, Video Volume dialog pop-up
5. On Video Volume dialog, uncheck the box for Mute, then move the Indicator to the middle of slider bar
6. Play this project

ACTUAL RESULT
The volume of the video didn't change at all

EXPECTED RESULT
It will turn down the sound