Lightroom can play any .mp3, .m4a, or .m4b music file as a soundtrack to a slideshow. Music plays as the slideshow is viewed from within Lightroom or exported as a video. Music does not play in exported PDF slideshows.
Also please make sure that quick time player is up to date.
Here is a reference link of slideshow in lightroom : Adobe Photoshop Lightroom Help | Play and export slideshows
This sometimes happens with DRM protected tracks. You could try converting to AAC format.
Also note that if you have ripped your own CD’s into iTunes it’s possible to create a duplicate in MP3 format which should work in Lightroom. See this thread:
I converted it to AAC, but Lightroom does not support AAC, ....
Tried everything, but nothing seems to work. Is it maybe useful to delete Lightroom and download it again?
As a process of elimination try with a different track. Or convert another track from AAC to MP3 and try that.
For another reason a had to restart my computer. After that I tried it again without any change of the settings and suddenly it worked
perfect! So I think the restarting of my Mac was the solution.
Thanks for helping me