I am familiar with the help info regarding slideshows. As I said earlier, I have made dozens of them in the past with earlier versions of LR. Now, when I press the plus button in the menu for adding music to a show, only a blank screen appears. There is no access to ITunes as I have experienced in the past.
Try playing it with the Default Template and a create a new slideshow(with 5 images)
To update QuickTime See Installing QuickTime Player 7 on your Mac - Apple Support
Also uncheck the "Use graphics Processor" under Preferences>>Performance and see if that helps
No success with your suggestions. There is a real problem with LR 6.3 on this issue.