Yes. Your background music is probably on track 3 or 4. Right click on it, select Volume Curve, Show. This will show the curve. Accept. Drag the up/down level control on the background music track to the volume that you want.
Note that this is done in the program as part of the creation process. Once you export the slideshow, everything is fixed in the output file. Whatever sound settings, fades, etc., that you set up in the making the slideshow cannot be changed during playback outside of the program.
Were you able to decrease the volume of the track in PhotoStory after creating the slideshow from the Slideshow Maker? See pages 114 and 115 in the manual (I only have 2013) about Automatic Volume Damping and Manual Volume Decrease. I had no trouble decreasing the volume of the background track that I added.