Mapping the audio channels in clips determines the type and number of audio tracks in which they appear in a sequence. Also, mapping channels determines their destination channels within the master track, and therefore in the final output file. For example, if you map channels 1 and 2 in a stereo clip to the Left-Front and Right-Front channels in a 5.1-channel master track, the two source channels appear as a single 5.1-channel track when placed into a sequence. They feed the Left-Front and Right-Front channels of the master track. When the final output is played through a 5.1-channel surround sound system, the original two channels play through the Left-Front and Right-Front speakers, respectively.
To learn more about mapping audio channels, see the video tutorial, Audio Channel Mapping In Premiere Pro CS3 on the Creative COW website.
Karl Soule shows how to map audio channels in Premiere Pro in this video tutorial. This video is for editors dealing with footage using split-mono tracks (voice on one track, ambient sound on another). This quick tutorial on audio channel mapping is a must-see.
By default, clip audio channels are mapped to the master track when they are captured or imported into a project. You can specify how Premiere Pro maps them by selecting a Default Track Format in the Source Channel Mapping pane of the Audio Preferences dialog box (Audio Channels in Premiere Pro CS5.5 and later). You can also change how clip audio channels are mapped after importing them into a project.
Finally, you can specify which output channels Premiere Pro uses to monitor each audio channel. For example, you can monitor the left channel of stereo tracks through the left-front speaker of your computer speaker system. Set this default in the Audio Output Mapping Preferences dialog box.