Tweak understanding for 1.2.3 and 1.2.5

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 may 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.

understanding/20/audio-description-or-media-alternative-prerecorded.html

@@ -20,9 +20,12 @@ Intent of Audio Description or Media Alternative (Prerecorded)
 
       One approach is to provide audio description of the video content. The audio description
          augments the audio portion of the presentation with the information needed when the
-         video portion is not available. During existing pauses in dialogue, audio description
-         provides information about actions, characters, scene changes, and on-screen text
-         that  are important and are not described or spoken in the main sound track.
+         video portion is not available. Audio description provides information about actions,
+         characters, scene changes, and on-screen text that are important and are not described
+         or spoken in the main sound track. Audio description is usually added during existing
+         pauses in dialogue. In rare cases where the original audio does not provide sufficient
+         pauses, it may be possible to create them by lowering the volume ("ducking") some of the
+         existing dialogue, if the need for audio description is deemed to be more important.
       
 
       The second approach involves providing all of the information in the synchronized

understanding/20/audio-description-prerecorded.html

@@ -16,12 +16,22 @@ Intent of Audio Description (Prerecorded)
       The intent of this Success Criterion is to provide people who are blind or visually
          impaired access to the visual information in a synchronized media presentation. The
          audio description augments the audio portion of the presentation with the information
-         needed when the video portion is not available. During existing pauses in dialogue,
-         audio description provides information about actions, characters, scene changes, and
-         on-screen text that are important and are not described or spoken in the main sound
-         track.
+         needed when the video portion is not available. Audio description provides information about actions,
+         characters, scene changes, and on-screen text that are important and are not described
+         or spoken in the main sound track. Audio description is usually added during existing
+         pauses in dialogue. In rare cases where the original audio does not provide sufficient
+         pauses, it may be possible to create them by lowering the volume ("ducking") some of the
+         existing dialogue, if the need for audio description is deemed to be more important.
       
 
+      
+         This success criterion applies even in cases where the original audio does not
+            have sufficient pauses, and where no portion of the original audio can be omitted in
+            favour of an audio description without losing important information. In these cases,
+            the content will fail this success criterion.
+         
+      
+