If you are using backing tracks in the OnSong Audio Library, you have the ability to adjust the volume and pan for each individual track.
To do this, open the Utilities Menu by tapping on the gear icon in the Menubar and then choose Editors > Audio Media Library. From here you will get a list of tracks that you've imported. Tap on a track to open the Track Editor. Locate the Volume Adjustment and Pan Adjustment sliders and change as needed. You can also use the Preview section to listen to the changes in real time. When you're done, tap Done.
You can also make changes to the audio track when using the Audio Playback option of the Heads-up Display.
Here's a tutorial video that also demonstrates how to manage your OnSong Audio Library.