You'll find further information about how to create slide shows with music under following link:
Creating slide shows with music

In Advanced Editing: 

If you want to adjust the volume of an audio track, move the mouse to the volume button on the left side of the audio track, right click and adjust.

If you want to adjust the volume of a specified video or music, select the video or music, adjust the audio level in top right Effect Palette -> Properties ->Audio Level.

In Express Editing: 

Move the mouse to the lower right corner of a video or music, the volume level button will show up.