Created a basic test slide show with a few jpg's and added some music. Exported this to 720p mp4, which only took a short time. Once completed, tried to run it, which it did, but showed the first slide only. The slide show continued, but with a black screen. Tried a few more, with different photos, all with the same result.
After that used LR 5.7 to create the same slide show and this worked perfectly.
As I used LR CC to create a lengthy slide show previously, the cause must have been since the last update a couple of days ago.
I wonder if anyone else has the same problem
It's working OK for me at 1080p. All slides play, although I haven't tried with music.
1) Did you have all of the JPEG image files selected in the filmstrip and not just the first one?
2) Close LR, reopen LR, try again.
3) Reset LR Preferences file. Resetting the lightroom preferences file - updated - Lightroom Forums
Just in case it was a fluke, tried another set of slides, this time without music. There were 12 slides, with 7 dng and 5 jpg. The slides used were from a Julieanne Kost exercise, so they should be ok. The resultant movie made showed the first 3 slides, then just blank screen. Will try the same on my notebook install later.