Close GH-720: unmute if the user drags the volume bar slider.

src/js/control-bar/volume-control.js

@@ -76,6 +76,10 @@ vjs.VolumeBar.prototype.createEl = function(){
 };
 
 vjs.VolumeBar.prototype.onMouseMove = function(event) {
+  if (this.player_.muted()) {
+    this.player_.muted(false);
+  }
+
   this.player_.volume(this.calculateDistance(event));
 };