Compressor About audio channels
Most of the audio settings in Compressor provide mono and stereo (left and right) channel configuration; some provide additional channel output options, including surround (5.1) channel configuration. Several of Compressor’s settings offer multiple audio channel configurations:
QuickTime Movie settings: QuickTime Movie settings include additional audio channel layouts, including 3.0, 4.0, and 5.1.
Dolby Digital audio settings: Dolby Digital (AC-3) settings include even more audio channel layouts, including 3.0, 3.1, 4.0, 4.1, and 5.1.
The illustration below shows the available audio channel layouts. 
The channel codes describe the location of the channel:
L: Left front channel
R: Right front channel
C: Center front channel
Ls: Left surround channel
Rs: Right surround channel
Cs: Center surround channel
LFE: Low-frequency effects channel (subwoofer, LFE)
Note: It’s important to understand that Compressor cannot convert stereo audio files into 5.1 surround sound channels. If you want to output a 5.1 surround sound audio file, you must first create the six channels of audio required and then import those surround sound files into Compressor. Then you can use Compressor to output the surround sound source media file. For more information, see Import and modify surround sound files.
After you import a surround sound source file, you can transcode that file into any of the channel layouts above.
Compressor analyzes audio files during import to determine the source file’s channels. When you apply a transcode setting to the source file, Compressor automatically adjusts the channel layout of the setting to provide logical results. For many settings, you can choose Automatic from the “Channel layout” pop-up menu (in the Audio inspector) to have Compressor determine the logical channel layout based on the source file.
Compressor also maps channels intelligently during output. For example:
If you assign a mono channel layout to a stereo source file: The output file will be automatically downmixed.
If you assign a surround channel layout to a stereo source file: The output file will have only left and right channels—mapped to the left front (L) and right front (R) channels of the source file—while the other channels are unassigned.
If you assign a left front or left rear surround channel to a stereo source file: Compressor maps the source file to the left channel (and ignores the right channel).