Issue with audio for slideshow with Photos
I'm not able to reproduce what you are saying. The duration in the top right under export seems responsive. I add the song I want, notice how long it plays and the slide show duration updates.

My guess is you have more than one song queued in the top right under selected music, but it's hard to know without poking around your project.



 The above is from Version 1.1 (330.21.0)