For those used to editing their audio channels in Final Cut Pro, Adobe Premiere Pro is slightly different. On the timeline the channels are merged into one track and not a left and a right. To edit the left and right you must double click the audio to bring it up in the Audio Mixer tab, here you will see this window.

If your edit requires further audio mixing make sure you change this setting at the point of setting up the sequence from stereo to 3.1, 5.1 etc

Your Audio Mixer tab will then look like this,