Expand audio description note to mention lowering of bg music/sounds

guidelines/terms/20/audio-description.html

@@ -9,8 +9,8 @@
       other visual content.
    
 
-   In standard audio description, narration is added during existing pauses in dialogue.
-      (See also extended audio description.)
+   In standard audio description, narration is added during existing pauses in dialogue. Usually, this will also require
+      lowering the volume of background music and other non-informational sounds. (See also extended audio description.)
    
 
    Where all of the video information is already provided in existing audio, no additional audio description is necessary.