When you add your video onto the timeline,look around the program interface for a symbol that looks like a mini 'loudspeaker'.(similar to the Volume Control Icon on your PC.) This offers the option to "Mute" the sound playback. Alternatively,again on the timeline,on the Audio section of your inserted video,drag the volume bar all the way down to the bottom. Then,on another track,insert your new sound source. Hope this helps!!??