allow to add different sound file for each slide

Right-click on the header of the Slide List | Details view (with the column names) and select the "Sound (slide-specific)" item in the popup menu to display the corresponding column. The Sound column allows you to add different sound files/playlists for each slide.

Also it's possible to use the Sync Playlist (the Sound button) to add different sound file for each slide and allocate slides using their duration setting. In this case the Background Playlist (the SlideShow | Background Playlist menu) can be used for background music.