Revert change to AD definition

Author: patrickhlauke

guidelines/terms/20/audio-description.html

@@ -9,8 +9,8 @@
       other visual content.
    </p>
 
-   <p class="note">In standard audio description, narration is added during existing pauses in dialogue. This may require
-      lowering the volume of background music and other non-informational sounds. (See also <a>extended audio description</a>.)
+   <p class="note">In standard audio description, narration is added during existing pauses in dialogue.
+      (See also <a>extended audio description</a>.)
    </p>
 
    <p class="note">Where all of the <a>video</a> information is already provided in existing <a>audio</a>, no additional audio description is necessary.