how to loop music in slideshow
How to force iPhoto slideshow to loop selected track from music
MacBook Pro 15″, macOS 10.13
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What system are you running?
How many slides in the slideshow?
How long is the music track?
With Ventura the music automatically loops until the slideshow is complete. I created a test slideshow with 28 slides. I set the playing time to 4 min 40 seconds (about 10 seconds per slide). The music track was 1 minute long. It repeated until all slides had played.