When I first ever used Sony Vegas I ran into this almost immediately, I wanted to be able to remove the audio track and replace it with another but every time I hit the delete key with the audio selected on the timeline it would delete the video clip too. This is a very simple tutorial so it wont really require a video but if anyone makes one specifically for this let me know and I will add it to this post. First things first, with your clip on the timeline right click the clip like so: After right clicking look for group, hover it with your mouse and then click on Remove from. You can alternatively click the clip in the timeline and hit U on your keyboard which is quicker. This will remove it from the group, when you add a video clip with audio to the timeline theyre automatically grouped. Now that they are ungrouped you can now delete the audio by clicking the audio clip and hitting the delete key on your keyboard. You will now have successfully separated and removed the audio from the clip. You can now add your own audio to the audio track or have the clip silent. I hope this has helped you.