Encore is no longer supported by Adobe. In Aperture this task is a piece of cake as it links directly with iTunes. A single track is useless in a lengthy slideshow. Surely there is a way to do this within the Lightroom slideshow module but I am unable to find it.
If you want to use multiple tracks then it will be necessary for you to use another program to integrate all of the music tracks into a single track to be used in Lightroom. Or, you can use other software to create your entire slideshow, a program that will accommodate multiple music tracks.
Use Audacity to join tracks and then use those joined track in LR.
It's available for Windows/Mac/Linux and is free.
Brilliant. Thanks for your help. Nicholas