media sound clip

Is this an audio clip you have added to a project or is it a video you have already with sound? If video uploaded with sound then it can't be deleted, only muted via the sound panel that open on the left-hand side. 

To open the sound panel in your project window click the button Edit Timeline. any sound clip added should show below the graphics as a blue line. Click on that blue line and use your Delete key to remove the clip and leave your graphics in place.