fixed by Qijia Yu <1131064840@qq.com> and Guanghao He <1477070788@qq.com>

<page xmlns="http://projectmallard.org/1.0/"
      type="topic" style="task"
      id="sound-volume">
  <info>
    <link type="guide" xref="media#sound"/>

    <credit type="author">
      <name>Phil Bull</name>
      <email>philbull@gmail.com</email>
    </credit>

    <credit type="author">
      <name>Shaun McCance</name>
      <email>shaunm@gnome.org</email>
    </credit>

    <revision pkgversion="3.4.0" date="2012-02-19" status="outdated"/>

    <desc>Set the sound volume for the computer and control the
    loudness of each application.</desc>
    <include href="legal.xml" xmlns="http://www.w3.org/2001/XInclude"/>
  </info>

<title>Change the sound volume</title>

  <p>To change the sound volume, click the sound icon in the status menu on the top bar that you are using (it looks
 like a speaker) and move the volume slider left or right. You can completely
 turn off sound by dragging the slider all the way to the left.</p>

  <p>Some keyboards have keys that let you control the volume. They normally look
 like stylized speakers with waves coming out of them. They are often near the
 "F" keys at the top. On laptop keyboards, they are usually on the "F" keys.
 Hold down the <key>Fn</key> key on your keyboard to use them.</p>

  <p>Of course, if you have external speakers, you can also change the volume
 using the volume control on the speakers themselves. Some headphones have a
 volume control too.</p>

<section id="apps">
 <title>Changing the sound volume for individual applications</title>
 <p>You can change the volume for one application, but leave the volume for
 others unchanged. This is useful if you're listening to music and browsing the
 web, for example. You might want to turn off the volume in the web browser so
 sounds from websites don't interrupt the music.</p>
 <p>Some applications have volume controls in their main windows. If your
 application has one of these, use that to change the volume. Otherwise, click
 the sound icon on the top bar and select <gui>Sound Settings</gui>. Go to the
 <gui>Applications</gui> tab and change the volume of the application there.</p>
 <p>Only applications that are playing sounds will be listed. If an application
 is playing sounds but isn't listed, it might not support the feature that lets
 you control its volume in this way. In that case, you can't change its volume.</p>
</section>

</page>

Yu Qujia:
Thanks for working on this! Could you turn your last comment into a patch file (which only contains your changes compared to the current version of that help file)? 
Please see https://wiki.gnome.org/Git/Developers#Contributing_patches for how to do that, and please ask if something is unclear with that documentation. Thanks!

line 20 add the on the top bar

Yu Qujia: Hi, did you mean to attach a patch, because we can't see it yet! :)

What we should do is change instructions to access the volume. If it's right,our change is add the on the top bar at the line 20.

Thanks for the suggestions. I reviewed the page and pushed the change to master as c1bc42fc663c68ed9840a37218a10d1a92c3784d.