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.
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. 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.