title: Converting 6 audio channels to 2 in ffmpeg
body: <p>Im doing a 2 pass encode on the uploads to my site. </p>
  
  <p>Unfortunately when a user uploads a 6 audio channel .mkv files (5.1 surround), ffmpeg spits out the following error:</p>
  
  <pre><code>Resampling with input channels greater than 2 unsupported.
  </code></pre>
  
  <p>Im running the latest version of ffmpeg. Doing <code>-ac 6</code> instead of the usual <code>- ac 2</code> proceeds with the encoding, but it just creates a larger file with useless data, as well as creates another step in the already long file processing script, and I'd rather avoid modifying the conversion command. </p>
  
  <p>Is there anything I can do to make <code>-ac 2</code> actually work?</p>