In the storyboard if you just click to add a sound track and then find the track that you want to add, and open it, you should get a box pop up saying background audio already applied do you want to replace it. If you click no then the new track will be added to the end of the already existing one.
You can then adjust the duration of the show fo fit the total soundtrack, or edit the sound track to fit the existing time of the show.
I have just recently completed several slideshows using Roxio Easy Media Creator Basic DVD, so I think I know my way around it reasonably well. If you want any further help just post back.