There is no answer to your question. It just can't be done that way in PD.
You have these options, if you want multi-track slideshows:
1. Mix the audio separately (outside PD) to create a single audio file containing your selected music. In Slideshow Designer, select your pre-mixed audio track.
2. Create the slideshows
without music - then add your selected music to one of the audio tracks. These can be mixed right in the timeline (splitting & removing, fading in & out)
Cheers - Tony
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