In my previous post, I discussed creating split-track audio – also know as stems – for the dialogue, sound effects and music components of the composite stereo mix. One useful aspect of the QuickTime format is that it can be a multi-track file, holding numerous discrete audio tracks within the same file. Likewise, Apple Final Cut Pro can create and use multi-track audio in a discrete fashion. The trick is in how you set up your sequence settings and in how you use the mixer panel.
Set the sequence audio configuration to Channel Grouped or Discrete Channels. This lets you control the output destination of your audio channels and whether they work as stereo pairs or as individual mono tracks.
In the Audio Outputs tab, establish how many target outputs the sequence will have. If you want three separate stereo pairs for dialogue, sound effects and music, then this tab should be set to six outputs of stereo pairs or dual mono tracks. If you are using stereo instead of multi-channel audio hardware (an Avid Mbox2 Mini in my case), you’ll receive a warning message alerting you that all tracks cannot be monitored. Just ignore it.
The last step is to make sure that your new sequence is actually set to output to the assigned tracks. Right-click on each track of the track panel and make sure your audio outputs are properly assigned. A1 and A2 to 1 & 2, A3 and A4 to 3 & 4 and A5 and A6 to 5 & 6.
Edit the stereo stem files to their appropriate tracks.
Notice a separate meter bar for each output track in the master section of the Audio Mixer. At this point you will only hear the output of audio track A1 and A2, due to your stereo audio hardware.
To monitor the composite mix, enable stereo downmix in the master section. Now all tracks are monitored. 
Muting and soloing specific tracks will let you isolate parts of the mix to hone in on a section. Working with stems can be very useful when the client calls to say they liked the mix, but can you bring the music down a bit. Instead of having your outside audio studio remix the track, simply make the level adjustment using these stems.
To archive your master file with discrete, split-audio tracks, export a self-contained file using Current Settings.
You can check this file in QuickTime Player 7 (Show Movie Properties) and verify the separate sound tracks embedded within the file.
In addition, you can import this file back into FCP, edit it across to a new sequence and confirm that the tracks are indeed discrete.
If you did make level changes to create a new mix from the stems, then it is also possible to export a self-contained version of the file with this new composite stereo track. Duplicate the sequence and change the settings back to a two-channel output. Make sure all track assignments are reset to 1 & 2. A self-contained export from this sequence will contain a single mixed stereo track.
You might also want to revisit “Sitting in the Mix” for more on mixing strategies.
©2011 Oliver Peters
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