Tweak understanding for 1.2.3 and 1.2.5

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.
-      (See also <a>extended audio description</a>.)
+   <p class="note">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 <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.

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

@@ -20,9 +20,12 @@ <h2>Intent of Audio Description or Media Alternative (Prerecorded)</h2>
 
       <p>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.
       </p>
 
       <p>The second approach involves providing all of the information in the synchronized

understanding/20/audio-description-prerecorded.html

@@ -16,12 +16,22 @@ <h2>Intent of Audio Description (Prerecorded)</h2>
       <p>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.
       </p>
 
+      <div class="note">
+         <p>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.
+         </p>
+      </div>
+
       <div class="note">
 
          <p>