Two ways:
1: tools->audio levels - the slide adjusts between the movie sound and
the audio track
2:
- In timeline mode: (see the icons at the bottom half of the screen)
- expand the video line (click the plus next to video at the bottom of
the screen) - you should now see two audio lines The top one is related
to the video and the bottom one is the music/voice over track
- right click either of them and adjust the volume:
1 is easiest but has the least control
2 is useful where a track is too quiet (so ajusting the ratios makes the
resulting movie too quiet) or where finer control is required.
Hope this helps
Rob