Here is what I do in order to lower the music track volume during narration or when some one is speaking in the video track.

1. Import the music to track 2.
2. Put the marker at the very beginning of the music.
3. Edit->Copy   Edit->Paste.
4. You now have two copies covering each other. Drag the top copy up to track 1.
5. Lock both tracks (Advanced->Lock Audio).
6. Highlight track 1 and lower its volume level half way.
7. On track 2, find the spot where you want the music lowered and split the AUDIO TRACK.
8. Now find the spot when you want the music back to full volume and split the AUDIO TRACK again.
9. You now have your music cut into 3 clips. Mark the middle clip and MUTE it.
10. Double click the first clip, mark and adjust FADE OUT.
11. Double click the third clip, mark and adjust FADE IN.
12. The MUSIC on track 1 is now the low level music.
13. Play your movie with TRACK 1 marked, Adjust the low level on the fly when you hear the narration or the wanted sync audio.
14. When you are done, (Edit ->Select All) and lock all audio clips (Advanced->Lock Audio).
15. Last cool step.  Drag down track 1 on top of track 2. And you will still have a free audio track!