If you are trying to stagger audio levels as you might with sound effects and a soundtrack then you are going to have a much more difficult of time. Here you really are going to have to use trial and error when listening. The big part here, especially with outside music, is that you are going to have to change the levels over the course of the audio clip. Your primary audio will change intensity, as will the song, so you have to go through and use Timeline markers to indicate these highs and lows. Make sure your Timeline markers are correctly labeled. From there you can move the audio levels up and down for your secondary track, and occasionally for the primary.