title: FFmpeg: How to merge all audio streams into stereo
body: <p>I have 4 audio streams in my video file. They are from 4 microphones placed at 4 different people speaking. I want to transcode to a preview file that can be listened to on headphones where everybody's voice can be heard.</p>
  
  <p>I have seen the -ac 2 option, but I can't tell if this will merge all the audio streams or just select the 1st two. I've also seen the amerge filter, but the docs say this will produce 4 audio channels in the output file. So I'm wondering how headphones will deal with the additional 2 channels</p>