This Audio menu under the Clip option offers other options for working with active audio clips in the Timeline. Depending on how each clip was recorded there are going to be variable volumes throughout each clip. This is an inevitable part of digital video production, but you may want each clip to be fairly even in its audio output in the final product. Inversely, you may want to have similar clips vary wildly in their volume position. Windows Movie Maker makes this easy in the same Audio menu that you selected mute from. Go to the bottom of this menu and find Volume, which is also just Control and U in the way of keyboard shortcuts. Make sure that the specific clip you want is selected in the Timeline before doing this. When you do this a windows will pop up with a slide scale for your volume, and the marker will be positioned in the middle to begin with. Now you can do this to each clip and adjust to where you would want it to be. There is also a mute check box here in case you change your mind about the audio clip altogether and did not want to go back to the audio pull down menu.