UPDATED: It's only one song, probably is a bug, . You can check older posts- 5-6 years ago for example, and even adobe employees say "1 track only" - and suggest using an external editor to make the music track, using multiple songs.
Personally I find the slidshow maker in organizer virtually useless.
Glenn, it is certainly a bug. The slideshow feature in the Organizer is completely changed from 5 or 6 years ago and has the ability to contain multiple different songs which can be added in the Audio panel.
I have been able to quickly reproduce this problem with Elements 2021. But I will test the issue a little further to see whether it is related to a theme or a particular kind of audio. I will report back.