title: Transcode audio from stereo .mp4 file into mono WAV 16KHz using MediaComposition
body: <p>I am trying to use the Media Composition and Windows.Media.Transcoding APIs to automatically convert the audio track from some .mp4 files I have into mono PCM WAV audio at 16KHz in order to use the Microsoft Speech cognitive services (Speech to text).</p>
  
  <p>I have a sample audio file with the right MEdiaEncodingProfile and I use <code>MediaEncodingProfile.CreateFromFileAsync(sampleAudio)</code>.</p>
  
  <p>After that, I set up a transcoder and then use </p>
  
  <p><code>PrepareTranscodeResult prepareOp = await transcoder.PrepareFileTranscodeAsync(SourceVideo.VideoFile, tempFile, profile);</code></p>
  
  <p>... but that results in <code>prepareOp.CanTranscode = false</code> since I do not think I can directly transcode a .mp4 into an audio file. </p>
  
  <p>Is there a way for me to just grab a reference to the left audio track in an .mp4 file and then transcode that out into a wav file?</p>