title: gstreamer pipeline only generates mono stream
body: <p>I'm trying to get UPNP streaming to work. <a href="https://live.gnome.org/Rygel%E2%80%8E" rel="nofollow">Rygel</a> runs fine, however, all I get is a mono stream, even if the input is stereo. Doing some debugging, I replicated Rygel's gstreamer pipeline with</p>
  
  <pre><code>gst-launch-1.0 pulsesrc device=upnp.monitor num-buffers=100 ! audioconvert ! lamemp3enc target=quality quality=6 ! filesink location=test.mp3
  </code></pre>
  
  <p>where the problem is also apparent:</p>
  
  <pre><code>mp3info -x test.mp3
  ...
  Media Type:  MPEG 1.0 Layer III
  Audio:       Variable kbps, 44 kHz (mono)
  ...
  </code></pre>
  
  <p>Where does this pipeline lose the second channel? How can I debug this?</p>